Grammar, even in our extremely confused language, is not particularly difficult. It consists of learning and remembering a group of relatively simple rules. However, the number of people who never bother to learn them is startling. I’ll outline the most commonly mistaken ones and provide examples of how to use them correctly. Learning all or even most of these will make you a much better writer - without correct grammar, it doesn’t matter how good your story is.

Two Subjects + Two Verbs = Comma needed.
    Eltea (Subject 1) wrote (Verb 1) a story, and Tierfal (Subject 2) edited (Verb 2) it.
One Subject + Two Verbs = No comma.
    Eltea (Subject) wrote (Verb 1) a story and edited (Verb 2) it.

When it is just one subject doing two things, all you need is an “and”. When two subjects are each doing something different, you need a comma. If you can remove the “and” and make two complete sentences, you need a comma. (i.e., “Eltea wrote a story. Tierfal edited it.”) However, if the second part is a fragment, no comma is necessary because it’s not a complete thought. (“Eltea wrote a story. Edited it.” would not work.)

Plural + Possessive = Don’t add an s.
    The conspirators’ plan
Noun/name ending in s + Possessive = Add an s.
    Brutus’s plan

Most people don’t have a problem with the first part. If the kids have a pet, it’s the kids’ pet, not the kids’s pet. However, what most people don’t realize is that this only applies to plurals. If a name ends in s, you can still add another - it could be Sam’s pet or Julie’s pet, so why not Carlos’s pet? The only names this rule does not apply to are Jesus, Moses, and a few others - it’d be Jesus’ plan or Moses’ pet, because the English language can’t have a rule without exceptions. ;)

Active Voice: Subject acting directly on object.
    Aragorn (subject) attacks the orc (object).
Passive Voice: The subject is acted on, usually using the verb “to be”.
    The orc (object) is attacked by Aragorn (subject).

In active voice, the subject is doing something. In passive voice, something is happening to an object. Active voice is generally preferred because it’s clearer and more direct. :)

Correct: Each subject is in agreement.
    Eltea’s and Tierfal’s story
Incorrect: Only one subject agrees.
    Eltea and Tierfal’s story

In the first example, it is a story belonging to both Eltea and Tierfal. In the incorrect example, the phrase is actually referring to two separate things: Eltea firstly, and Tierfal’s story secondly.

Correct: Each author will write his or her own story.
Incorrect: Each author will write their own story.

This is one of those rules you just have to learn.

Correct: Verb immediately follows “to”.
    I plan to write a story someday.
Incorrect: Another word is between “to” and verb.
     I plan to someday write a story.

Make sure that when you use the “to ____” form of the verb (which is called the infinitive), you keep it like that without putting anything in between.

To begin with, Spellcheck is your friend. Let me say that again. SPELLCHECK IS YOUR FRIEND. If you have any kind of Spellcheck software available, you should never, ever show off your work - or even ask somebody to edit it - without having spellchecked. It only takes a few minutes, and if you want to be a writer, you have to do it.

Affect is a verb. Effect is a noun. If something affects you, it has an effect on you.

“All right” is two words. “Alright” is slang.
“A lot” is two words. “Alot” is incorrect. “Allot” means “to give or apportion”.

“Awhile” is an adverb, which means it describes a verb.
Correct: We talked awhile, or We talked for a while.
Incorrect: We talked for awhile.

They’re means They are. (They’re nice people.)
Their is possessive. (I saw their cat.)
There is a place. (They live over there.)

It’s means It is. (It’s a dog.)
Its is possessive. (It chases its tail.)

1. Netspeak - it’s 4 im-ing, not 4 serious writing lolz!
2. The filler “like” - It’s, like, totally superfluous, and, like, pretty annoying too.
3. Slang - It’s totally not awesome. Neither is it professional.
4. Mid-Story Author’s Notes - They’re [AN - See, I used “they’re” right!] extremely distracting.
5. Lower Case: if i don’t care enough to capitalize, i shouldn’t be writing at all.
6. Caps Lock: ONE WORD IS DRAMATIC. AN ENTIRE PARAGRAPH IS ANNOYING.