The more you get right in your manuscript before submitting to editors, the less work they'll have to do on mechanical issues—and the more time they can spend on making your writing shine. Plus, learning the conventions of fiction style* can save you money.
*This is Articulate Editing house style, which is based on generally accepted fiction conventions (in the US) and a pinch of personal opinion. Most of what is written here will apply across the board. If a guideline has wiggle room, I'll let you know. And if you're working with a publisher, make sure you follow their specifications as their house style may differ slightly. (This style guide also applies to creative nonfiction.)
Standard plural forms
To form the plural of most English words, stick an s on the end. Unless the word ends with s, j, z, x, ch, or sh, then add es. But there's lots of exceptions, so if you're unsure of the correct plural form of a word, check Merriam-Webster's Collegiate Dictionary or Merriam Webster's Unabridged online dictionary. If there's more than one option, choose the first (unless the plurals are used for different purposes.)
To form the plural of words that end in y, change the y to ie and add s.
one try/two tries
Many, but not all, nouns that end in f or fe change to ve in the plural.
Words that end in o are inconsistent. With most you add s, with some es, and a few are correct either way. Check your dictionary for the preferred spelling.
calico/calicos or calicoes
zero/zeros or zeroes
Plurals of compound nouns
The plurals of many compound nouns will be in the dictionary. For those that aren't, use common sense: the more significant part gets made plural.
day laborer/day laborers
area of expertise/areas of expertise
Plurals with italics and quotation marks
To form the plural of a word inside quotation marks, keep the s inside the quotation marks.
Your speech was full of "like"s. X
How many "abracadabras" must I say? ✓
With Italicized words, the s is usually set in roman.
How many abracadabras must I say?
With plurals of foreign words that are italicized, italicize the whole word.
Plurals of letters, numerals, and abbreviations
Plurals of uppercase letters, numerals, and abbreviations are normally formed by adding the letter s.
I've seen three UFOs.
She earned all As.
The 1990s were a great decade.
To form the plurals of lowercase letters, add 's. Italicize the letter as letter, but leave the apostrophe and s in roman.
x's and y's
"How many s's in Mississippi?"
But ... these common expressions are set in roman.
mind your p's and q's
dot your i's and cross your t's
Plurals of proper nouns
Most plurals of proper nouns are formed by adding s, or es if the name ends in ch, j, s, sh, x, or z. Exceptions can be found in the dictionary.
the Chavez family/the Chavezes
the two Germanys (not Germanys)
How many Monets are in your art collection?
There are three Toms in my class.
Never use an apostrophe to form a plural of a surname.
The Chavez's are coming over. X
Names that end in a silent s or x are best left singular.
Two Joneses, two Delacroix, and three Deschamps are coming.
Collective nouns can be singular or plural.
Whether or not they take a singular or plural verb depends on whether the group is acting as a whole or as individuals.
My family is the best!
His family are constantly at each others' throats.
To form the possessive of most singular nouns, add an apostrophe, then an s.
the seat of the chair/the chair's seat
the elephant's trunk
This includes abbreviations and numbers.
the Glock 17's bullets
the UFO's blinking lights
And nouns that end in an unpronounced s.
the marquis's estate
This also applies to singular nouns (common and proper) that end in s, x, or z.
the boss's daughter
Mrs. Chavez's car
the fox's tail
But ... this is an area where styles vary.
Some publishers may prefer just an apostrophe after singular nouns that end in s, and independent authors can decided for themselves how to style singular nouns that end in s. Remember, euphony can trump rules if you end up with a phrase full of sibilance, like "Seamus's sustenance."
CMOS used to recommend that classical and biblical names ending in the "eez" sound take only an apostrophe, but now recommend adding 's.
Use your ear (and the dictionary). If common phrases don't include the extra s sound when spoken aloud, leave it out.
for goodness' sake
Other versions that follow the "for ... sake" pattern follow the general rule. Again, your ear will guide you.
for Jesus's sake
for appearance's sake
When forming a possessive of an italicized word, the 's should be in roman if the surrounding text is in roman.
To the Lighthouse's use of imagery
the letter m's double humps
Avoid forming possessives of words in quotations. Rephrase instead.
"The Raven"'s use of imagery X
Poe's use of imagery in The Raven ✓
To form the possessive of a plural noun that ends in s or es, just add an apostrophe.
your parents' house
This is true of surnames, as well. However, surnames tend to confuse people. If you're writing about two or more people, first form the plural (see above), then add an apostrophe.
Miss Chavez lives alone → Miss Chavez's home
two or more Chavezes share a home → the Chavezes' home
With irregular plurals (that do not end in s), add an apostrophe and an s.
his teeth's evenness
Words whose singular and plural form are the same, and end in s, take only an apostrophe in both forms.
that species' peculiarities
But ... it's often better to avoid the possessive by using "of."
the principles of economics
Places, organizations, and publications that end with a plural word take only an apostrophe, even though the entity is singular.
the United States' international reputation
Agoura Hills' parks
New York Times' editorial staff
With group names that imply ownership or participation, use the apostrophe unless it's left out of an organization's official name. However, this is a rule in flux, and different style guides handle this differently. So if you're self-publishing you can choose to leave out the apostrophe if the sense is less about ownership than participation. But check the dictionary—the apostrophes aren't optional in Mother's Day or President's Day.
farmers' market (or farmers market)
With compound nouns, the last word becomes possessive.
This jack-of-all-trade's versatility is legendary.
The attorney general's testimony is crucial.
Check out the farmers' market's offerings!
This is true even if the compound noun is plural.
Jacks-of-all-trade's versatility is apparent in the name.
the attorneys general's appointments (the appointments of more than one attorney general)
my sisters-in-law's attitudes (speaker has more than one sister-in-law)
If two or more people own something jointly, then only the last one listed takes an apostrophe.
Gerry and Martha's house.
Bob and Jim's boat.
If they possess the same category of things, but possess them individually, then all take apostrophes.
Sally's and Betsy's hair got blown around.
Bob's and Jim's clothes were hanging over the side of the boat.
The stand-alone possessive pronouns are mine, yours, his, hers, theirs, ours, and whose. The modifying possessive pronouns are my, your, his, her, its, their, our, and whose.
Possessive pronouns never take apostrophes. Because nouns do, it's easy to associate apostrophes with possession and get confused, especially with its/it's.
its = belonging to it
it's = contraction of "it is"
Other commonly confused possessive pronouns:
your/you're (contraction of "you are")
their/there (meaning "that place"/they're (contraction of "they are")
whose/who's (contraction of "who is")
our/are (present tense second person singular and present tense plural of verb "to be")
Possessives with gerunds
Gerunds, which act as nouns, are modified with adjectives—so traditionally, nouns and pronouns right before a gerund needed to be possessive modifiers. "Playing" and "making" are the gerunds in this example.
I really enjoyed Sally's playing.
I can't understand his making do with so little.
According to CMOS, using the possessive is optional after a preposition.