Rule

To form the plural of most nouns, simply add -s

Examples

cat > cats, dog > dogs

Exceptions

Surviving Old English plural forms: deer > deer; sheep > sheep; ox > oxen; man > men; woman > women; child > children; brother (in a religious sense) > brethren; foot > feet; tooth > teeth.
Some compound nouns: passer-by > passers-by; mother-in-law > mothers-in-law.
Plural forms of not fully naturalized foreign words: chateau > chateaux; crisis > crises; formula > formulae; index > indices; stimulus > stimuli
Plural forms of metric abbreviations: 100 km (kilometres); 60 g (grams); 2.5 l (litres)

Note that with non-metric measurements plural s is optional: 60 lb or 60 lbs

Rule

To form the plural of nouns ending in s, sh, ss, z, x or ch, add -es to facilitate pronunciation

Examples

gas > gases; dish > dishes; boss > bosses; box > boxes; watch > watches.

Rule

To form the plural of nouns ending in y preceded by a consonant, change the y to an i and then add -es

Examples

lady > ladies; baby > babies; strawberry > strawberries; laboratory > laboratories.

Exceptions

lay-by > lay-bys (BrE); stand-by > stand-bys.
Family names: Mr. & Mrs. Brady > The Bradys

Rule

To form the plural of a number of long established English nouns ending in f or fe (but not ff or ffe!) change the f to a v and then add -es

Examples

half > halves; leaf > leaves; life > lives; knife > knives

Exceptions

Most other nouns ending in f or fe simply add -s as usual, but there are some cases in which the -ves plural formation is optional:
belief > beliefs; chief > chiefs; handkerchief > handkerchiefs
(but note handkerchieves is an option in British English); safe > safes
Caution is advised and, if in doubt, consult a dictionary.

Rule

To form the plural of imported nouns ending in o and long established in English, add -es

Examples

cargo > cargoes; domino > dominoes; echo > echoes; embargo > embargoes; hero > heroes; potato > potatoes;
tomato > tomatoes; tornado > tornadoes; torpedo > torpedoes; veto > vetoes.

Exceptions

For less naturalized nouns ending in o, add -s only:
kilo > kilos; piano > pianos; kimono > kimonos; radio > radios.
In a few cases the -es plural formation is optional. When in any doubt, consult your dictionary!
archipelago > archipelagos/archipelagoes; fiasco > fiascos/fiascoes; halo > halos/haloes; mango > mangos/mangoes.

Rule

A final silent e is usually dropped before adding a suffix beginning with a vowel

Examples and exceptions

-able
It is not so easy to predict when a final e should be dropped before adding the suffix -able.
Generally speaking, this is more common in American English. In some cases it is optional:
likable or likeable

Words ending in -ce and -ge usually keep their e to clarify the pronunciation of a preceding consonantal sound:
noticeable, peaceable, knowledgeable, manageable

The final e is also retained for words ending in -ee:
agreeable, foreseeable

-age
store > storage
but acreage, mileage
Note that linage and lineage are entirely different words.

-ed
clone > cloned

-est
cute > cutest

-ing
dive > diving; write > writing
But you must retain the e when it is needed to distinguish meaning:
dying, dyeing; singing, singeing.
Sometimes the e is kept to clarify pronunciation of a preceding consonantal sound:
ageing
American English seems to be less particular about this:
aging
The final e is also kept for words ending in -ee, -oe, or -ye:
fleeing, canoeing, eyeing

-ous
fame > famous; pore > porous
Words ending in -ge keep their e to clarify the pronunciation of a preceding consonantal sound:
advantageous, courageous
For words ending in -ce this final e becomes an i:
space > spacious

-y
ice > icy; bone > bony
The final e is not dropped before adding the suffix -y if the preceding letter is u:
gluey but gluier.
Note also pricey and spacey.
Remember that holy and holey are very different words!