Ohayo: "Good morning/Hello." For a more polite greeting, use "ohayo gozaimasu."
Konnichiwa: "Good afternoon/Hello." Contrary to popular belief, this is not applicable at all times of the day.
Konbanwa: "Good evening." Not used often in fanfiction, but appropriate nonetheless.
Moshi moshi: "Hello." Only for the telephone; it is never used in a conversation otherwise!
Sayonara: "Goodbye." For when you don't think you will see the person again soon. An interesting note - do not use this on the telephone. People in Japan do not do it because it seems like you will not talk to the person again.
Sarabada: "Farewell." More formal than "sayonara."
Ja matta: "See you." This is used when you think you will see the person again that day.
Ja ne: "See you." The same as "ja matta." You can use both phrases on the telephone.
Ja matta ne: "See you (later)." A combination of the two aforementioned phrases. It is appropriate for the telephone and personal conversation.
Domo: "Thanks." Informal term, usually used when a person does you a small favor or between friends.
Domo arigato: "Thank you." More formal. (Not about Mr. Roboto!)
Domo arigato gozaimasu: "Thank you very much." Most formal.
Domo arigato gozaimashita: "Thank you very much (for what you have done)." This is a past tense term, and is very formal.
Sankyu: "Thank you." It stems literally from the English words. Some characters use it; many do not. Be sparing in its usage.
Doo itashimashite: "It's nothing/Don't worry about it/Think nothing of it." The Japanese way to say, "You're welcome." It's not proper to accept a compliment - you deflect it - and you do the same with thanks.
Gomen: "Sorry." Informal.
Gomen nasai: "I'm very sorry." More polite.
Sumimasen: "I'm sorry/I apologize/Excuse me (for getting a person's attention)." The most formal way of apologizing or getting a person's attention.
Hajimemashite: "Let's begin." This is the Japanese equivalent of, "It's nice to meet you."
Irasshaimase: "Welcome!" In Japan, the store clerk will say this to you when you walk in. It's said quite cheerfully.
Onegai: "Please." Normally said alone or combined with another phrase. Polite people use it, rude people don’t. Like in English.
Onegai shimasu: "Please." More polite. Some people will go for this rather than just "onegai," if they are more formal or respectful to others.
Kudasai: "Please." Part of a request. Polite people use it, and people use it when talking to someone superior to them. For an example of context, it would be used in sentences like, "Tatte (stand) kudasai."
Baka: "Idiot." Probably one of the most commonly known Japanese terms in fanfiction. Most Japanese believe this is a curse word, so use it sparingly.
Bakayaro: "Asshole/Bastard." Like with "baka," be wary of usage; Japanese people do not swear often in real life.
Kono yaro: "Asshole/Bastard/Jerk."
Yarou: "Asshole/Bastard/Jerk." Note that this is the same as in "bakayarou" and "kono yarou."
Kuso: "Shit/Damn." People use this more in anime than in real life, like all other curse words.
Ch': It is similar to "chikusho," and is likely a shortened version. Saying "ch'" is like saying, "Damn," or "Darn."
Kusotare: "Shithead." Some people think this means bastard. It does not. Kuso is shit; figure it out.
Nande kuso: "What the hell?"
Shimatta: "Darn/Shoot/Too late." Lighter cuss word. More commonly used than its ruder counterpart. However, its last meaning ("too late") is most commonly used.
Ecchi: "Pervert." This is somewhat akin to calling a person "perv." It is lighter than saying "hentai," but is still insulting. Its pronunciation comes from the Japanese way to say the letter "h."
Hentai: "Pervert." Not as common as calling someone a pervert in the United States, so it is indeed an insult and a serious accusation.
Hentai, te o dokete yo: "Pervert! Get your hands off me!" For all those female characters who need to pull mallets on the men of their series, this is appropriate. It also applies to situations such as being groped on the train.
III. Me and You
Watashi: "I/Me/Myself." Moderately formal, usually used by females.
Watakushi: "I/Me/Myself." Very formal and rarely used.
Atashi: "I/Me/Myself." A feminine word, often considered cute or flirty.
Sessha: "I/Me/Myself." Archaic. No one says it. Don’t use it unless you’re writing for Rurouni Kenshin, got it? Saying "sessha" is like saying, "this unworthy soul" or "this humble person."
Boku: "I/Me/Myself." Tough and considered antisocial for girls, usual for boys and young men. As women in Japan begin to become more feministic, some business women will use this, but it is rare.
Ore: "I/Me/Myself." Slangy and rather rude. People who throw around insults and get into fights are likely to use this.
Anata: "You/Honey (Lover)." Rather formal. Japanese people do not use this often with its first meaning, I might mention. They prefer to use your name. But it works in fanfiction context, so you can use it. Actually, it's smarter to use it as its second meaning.
Anta: "You." Less formal. Guys can use it in normal conversation, though again, Japanese people tend to use your name. Girls who use this are tough and somewhat antisocial.
Kimi: "You." Friendly, but not tough. If a guy uses "boku," odds are he uses this as well. Tough girls use it, too.
Omae: "You." Teenaged boys and young men use this because it's slangy. To people who use "ore," you are "omae." When using it with acquaintances, it is very disrespectful.
Temee: "You." Very, very rude. Normal conversations should never include it because it is far too disrespectful. It's like saying to somebody, "you piece of shit," or "you asshole." Males only.
Kisama: "You." Also very, very rude, but has more dignity in it. Anime dubs often translate it as "bastard," because it's like addressing someone as such. Still males only.
(o)kaa-san: "Mother." More formal using "-sama," nicer and cuter using "-chan." (Using "-san," it is like "mom;" "-sama" is like "mother;" "-chan" is like " mama" or "mommy.")
(o)tou-san: "Father." Can also use "-sama," or "-chan." (Similar equivalents apply.)
(o)nii-san: "Older brother." Again, "-sama," or "-chan," depending. Can also be used to address a man who is older than yourself, but not yet middle-aged.
otouto: "Younger brother."
(o)nee-san: "Older sister." Again, "-sama," or "-chan." Can also be used to address a female who is older than yourself, but again, not middle-aged.
imouto: "Younger sister."
(o)ji-san: "Uncle." Again, "-sama," or "-chan." Can also be used to address a middle-aged man.
(o)ba-san: "Aunt." Again, "-sama," or "-chan." For addressing a middle-aged woman.
(o)jii-san: "Grandfather." Again, "-sama," or "-chan." Used to address an older man.
(o)baa-san: "Grandmother." Again, "-sama," or "-chan." Used to address an older woman.
waifu: "Wife." This is more common now, and comes from the English word. Many men, even salarymen and seemingly traditional businessmen, use it.
nyoubou: "Wife." A more traditional word.
-chan: used for affection. Used from parent to child, girl to her (girl) friend, lover to lover, owner to pet. It is also a feminine term and should be used sparingly between males.
-san: respectful, standard suffix. Often for someone older than you, a parent, or someone that you do not know well. In English, one might equate it to "Mr." or "Ms."
-kun: term of affection. It was only used for boys for a very long time, but these days it can be used for girls (with their last name only).
-sama: highly respectful. For rulers, deities, or people you admire greatly.
-dono: very formal and implies the superiority of the person in relation to you. Some people think this is only for females. But watch those samurai movies and you notice that it was used to address lords in feudal times.
gumi: used to refer to the people who hang around somebody. For example, the "Kenshin-gumi" is like saying, "Kenshin's group."
-jin: the people in a group, sometimes race. The Japanese call foreigners "gaijin." You get the idea.
-tachi: is like saying, "and the others." So if you address someone and the people around them, it's "Yusuke-tachi," or "Yusuke and the others/everybody with him." Saying, "watashi-tachi" is like "we," just as saying "anata-tachi" is like saying, "you all."
-sempai: use this when the person is older than you and superior to you. In other words, if you call him "Keitaro-sempai," he is in a higher grade and more experienced than you. It’s the respectful thing to call someone who is such. On the opposite end of the spectrum, you are "kohai," or the inferior, though it is not a suffix added to your name.
-sensei: can be used for teachers, doctors, or instructors of other kinds (such as kendo or martial arts instructors). You can refer to the person as merely "sensei," or by their name, followed by "sensei."
-pon/-pi: very playful suffixes which tend to replace the last part of a name. They are often used by girls, but rarely in relation to their boyfriends. An example of what I mean would be from Campus Police Duklyon, by Clamp: "Takepon" and "Kenpi" (though they are both male). Use sparingly, but humorously.
(O)jou-chan: "Little girl/Missy." You can use it for a young girl or someone's daughter (not yours). Of course, as you may know, it can become a nickname in itself. (Think Sanosuke in Rurouni Kenshin referring to Kaoru as "Jou-chan.")
VI. Grammar issues
ka: at the end of a statement, it makes a question. Sometimes Japanese people do not use it, and just raise the tone of their voice. However, I think you'd better use it. An example is, "Daijoubu ka?" which means, "Are you all right?"
desu: used like a "be" verb. Desu is more polite. In Tokyo dialect, slang is "da." If the people don't live in Tokyo, were not born there, or do not tend to use slang, do not use "da."
de gozaru: it is like "desu," but very old. Nobody, and I repeat nobody, says it anymore. It's quite uniquely Kenshin's. I didn't know it existed until I saw Rurouni Kenshin.
ne: use it in context. At the end of a sentence, it can mean "isn't it/aren't you/correct." Japanese people sometimes use it alone to show agreement or interest. At the beginning of a sentence, it's like saying, "well." Addressing someone with it like, "Ne, Hikaru..." is like saying, "So, Hikaru..."
no: an article. Generally, it means, "of." It can also make things possessive (e.g., "Eriko no kasa" means, "Eriko's umbrella").
mo: a possessive. Saying "atashi mo" is like saying "mine" in English. You can also say "no," but this is not correct in many cases when "mo" should be used.
-ru: verb ending. To say "kotowaru" is to say that you are refusing. Kotowa is refuse, and adding -ru makes it a verb. When encountering the phrase "kotowa" below, remember this.
Aishiteru: "I love you." Some people consider it sappy; most Japanese do not say this.
Suki (desu): "I like you." Boyfriends and girlfriends say this to one another most of the time, as it isn't considered sappy and conveys a meaning somewhat equivalent to "I love you." The Japanese tend not to be very demonstrative, and this is evidence.
Dai suki (desu): "I really like you." Most Japanese are more likely to say this than to say "aishiteru." Again, even boyfriends and girlfriends say this instead.
Dai kirai (desu): "I really hate (you)." Obviously, this is no term of endearment, and is not often used. However, one may use it in relation to objects, such as food.
Koibito/Koi: "Sweetheart/Lover." Use this to address your lover, as in, "Ne, koi, ikimasho (let's get going)." Also use it to refer to them, as in, "watashi no koibito."
Tomodachi: "Friend." To say, "Kanojo wa tomodachi no *insert name here* desu," is like saying, "This is my friend, *name*." You can also say "your/my tomodachi," as in using a possessive like, "my friends."
Daijoubu ka: "Are you all right?" Exclude the "ka," and it becomes, "I'm okay/It's okay."
Omae o korosu: "I'm going to kill you." Gundam Wing fans know this one well. Heero is written into fanfiction as using it constantly.
Ganbatte yo: "Keep it up."
Itterashai: "Take care." Said when a person is leaving the house. Chobits fans, you will remember this fondly. Usually comes before the person says "itekimasu."
Itekimasu: "I'm going and I will come back." The person who is leaving the house will say this as they go out the door. It can be preceded by "hai," so it then means, "Yes, I'm going and I will come back."
Tadaima: "I'm home." The person who is returning says this as they reenter the house.
Okaeri nasai: "Welcome back." The person at home says this by way of a greeting to the person returning. Follows "tadaima." It can also be shortened to "okaeri."
Shou ga nai: "Can't be helped/Nothing you can do/No other choice." Expresses a very basic Japanese attitude that makes some Westerners call them fatalists.
Ii ja nai (ka): "Why not." Rather like saying, "Oh, what the hell," and it can imply an attitude of, "You only live once." It's a nice historical idea, too. During the last days of the shogunate, this became quite a popular attitude. People just went out on the road and had themselves a good time. Things were bad for everybody. So... Ii ja nai ka?
Shine: "Die!" No, that is not English "shine" as in "bright." It's a murderous statement. You may have heard it along the lines of "Shine, Battousai!"
Sugoi: "Cool." Another of the most popular Japanese phrases in fanfiction. Also can be used as "Sugoi, ne!"
Kawaii: "Cute/Pretty." Probably the most popular phrase, along with "baka," which strikes me as funny. Japanese girls and women are very fond of this word indeed. You can also say it, "Kawaii, ne!"
Ustukushii: "Pretty." This is usually used not to describe something, but to describe a person (in my experience) as being cute.
Kirei: "Beautiful." Beyond simply "kawaii;" this implies elegance, grace, and loveliness.
Bishounen: "Beautiful boy." Often seen in yaoi (and in het), this refers to a boy between the ages of twelve or thirteen to sixteen.
Biseinen: "Beautiful youth." Rarer than its counterpart, it is in reference to a young man from sixteen (more likely seventeen) and into his early twenties. It refers to samurai culture, in which the samurai were fascinated by - and often took them as lovers - bishounen, but rejected them as soon as they grew into young men.
Bishoujo: "Beautiful girl." One might think of "Bishoujo Senshi Sailor Moon" in this context. As you may have noticed, "bi-" is the prefix that means "beautful."
Kowai: "I'm scared/[That is] scary." This apparently is a big problem with gaijin. They mispronounce "kawaii," and say "kowai" instead. Not a good idea.
Sou ka: "Really?/I see." Raise your voice at the end and it is the former. Say it otherwise, and it is the latter. Sometimes, people may say, "Sou desu ne," like, "So it is/That's right/Really?"
Nani: "What?" Just what it sounds like. Quite a lot of people know this one, too.
Naze: "Why?" Ditto as for "nani," except fewer people use it in fanfics. I can't imagine why not.
Doushite: "How come?" This is like "naze," and frequently used in anime.
Oro: "Huh?" This is rarely used in Japan. People prefer "ara." It's a sound of confusion that I associate as one of Kenshin's pet phrases (in Rurouni Kenshin). Most characters will not use it, so for them, use "ara?" Others may also use "are?"
Yosh': "All right!/Let's go!/Great!" Romanize it with the apostrophe, as it is pronounced that way.
Oi: "Hey!" Like calling someone, or addressing them.
Mou: "Hey!" Don't get this confused with "oi." It's a protest. Should someone tease you, you can say, "Mou!"
Cho mukatsuku: "You/that tick(s) me off!" This one is a favorite of mine. I don't know anyone who has ever used it in fanfiction, but it's fun, so I thought I'd include it. Right now, it's all the rage for Japanese teenagers to say this when someone annoys them. "That teacher...cho mukatsuku!" is basically like, "That teacher really bugs me/pisses me off!" Pronunciation in saying it is important. They usually say, "Choooooo mukatsuku!" "Cho" is "very" or "so." Think, "That makes me soooooooo mad!" and you'll know what I'm talking about.
Hontou ne: "Really?" Can be sarcastic, like saying, "Yeah, right." But it can also be an honest question.
Hou na: "Really?" This is like saying "hontou," and is not sarcastic. But I warn you: it is from Yoben (Shikoku) dialect. Anyone much farther north than Osaka will likely not say it, although they will understand it.
Taihen: "Terrible/Tough/Hard." Can be used as in "Taihen ne," which is like saying that something is horrible or very, very bad. It can also mean that something is incredibly difficult. It depends on context.
Warui: "Bad/Wrong." For example, "Warui desu ne," or, "How bad." Then, the second meaning is more like "Nani ga yoku te warui no ka." This line (from The Real Folk Blues) translates roughly to, "What is right, what is wrong." Warui, obviously, is wrong.
Yame: "Quit/Stop." Can be an order, as in "stop moving," or a command to get someone to stop doing something.
Yamero: “Quit it/Stop that/Leave me alone/Don't!" This form is like a request. To say "yamero" is similar to saying, "Please don't." Polite people use this.
Yamete: "Quit it/Stop that/Leave me alone/Don't!" Same thing as "yamero," but more forceful. It's not a request. Like saying, "Hey, cut that out now!" Ruder people will use it.
Yametegure: "Please stop/Please don't!" The strongest form of "yamero," but it is also a plea. Don't use it as an order at the risk of being laughed at. Gentle and polite people say this, and rude people will not say it unless they're really worried. However, don't mistake it for begging. It is not.
Hai: "Yes." Probably everyone knew that. It's another common one.
Aa: "Yeah." The informal version of "hai," used for agreement or in response to a question. I'm seeing this more these days.
Iya: "No." The more common way to say no, used by itself.
Iie: "No." More formal. Kind people say it. Rude people tend not to. However, you don't typically use it to decline a request or to disagree
Chigaimasu: "It's different." That's the literal translation. This is used to decline something, and is more formal than its counterpart, "chigau," though they both mean the same thing. Usually preceded by "iie."
Dewa arimasen: "There is none/It doesn't exist." Often, this is the reply to a question. When you are asked if something is there and it is not, you would say "dewa arimasen."
Kotowa: "I decline/I refuse." Instead of shouting "iie" to everything, this is used. It is more polite and proper, and it carries a stronger meaning.
Kami(-sama): "God." Some people use it as an expletive, like "Aa, Kami-sama!" It also means "spirit" in Shinto, the traditional Japanese religion.
Maa maa: "Calm down/Oh, dear/Come now." It can mean anything similar to those statements. A character might say this if people are arguing or getting riled up about something.
Yare, yare: "Well, well/Oh, oh/Oh, well/At least that's over." Literally, the meaning is the last one. In anime, that is usually not the case. It's kind of like a reluctant sigh of giving in. When someone does something you don't like, but that you can't stop them from doing, you say, "Yare, yare." I have to say, though, that I've heard this used in the same context as "maa maa" and so far, I believe that this is also okay. Don't quote me on that. If anyone else knows differently, point it out to me.
Saa: "Well." Not as in how you are doing, but an interjection, separated from the sentence by a comma. It is rarely seen, but when you do use it, usually it will be in the context of something like this: "Saa, a true genius can do anything."
Hn: It's not really specifically Japanese, but it is often used in anime - even more often in fanfiction - and has no literal meaning. To respond to a statement or question, some characters - usually the more antisocial ones - will use it as a sort of noncomittal grunt.