How to use the apostrophe.

I wrote this when I was, like, 15. Tagged as Learn English!.

The apostrophe (’) is used for indicating possession (my friend’s dog); to form plurals of abbreviations (M.B.A.’s), characters (S’s) and dates (1970’s); and to show the missing letters in a contraction. When referring to whole words, its usage depends.

Showing possession

An apostrophe and an S are added to the end of a singular noun (cat, dog, son) if that noun has an object. For example:

Dad’s car is orange, but Mum’s yellow car is better.

If the noun in question ends with an S (yachts, readers, gods) an apostrophe is added without the S.

All the yachts’ hulls were damaged in the storm.
Readers’ Choice Award
The gods’ decisions are said to be just.

However, many print publications like newspapers and magazines use ’s when a proper noun ends in an S. This is for clarification and a question of style: what you do is up to you.

Jesus’s sacrifice
Athens’s Olympic preparations
Britney Spears’s sordid history

The possessive form of it never has an apostrophe (its). It only has an apostrophe when you mean to say it is (it’s).

Forming plurals

Apostrophes are often used to make plurals for abbreviations and characters, though not often correctly. The rules are:

Capitalised abbreviations do not require apostrophes.
If you can spell out a character's sound or name, do so.
Only use an apostrophe when the plural is unclear.

Pluralising capitalised abbreviations

Take for example, the plurals for CD and DVD, often shown in advertisements as

CD’S or CD’s
DVD’S or DVD’s

Removing the apostrophe makes the word look more attractive, and we can do it by keeping the abbreviation capitalised and making the S lowercase:

CDs
DVDs

Non-capitalised abbreviations (like laser) do not have apostrophes (lasers).

Pluralising characters

Typically when referring to characters you use apostrophes to clarify the word: S’s makes more sense than Ss. Learning the name of the character you’re referring to can simplify things greatly.

#'s becomes “hashes”
@'s becomes “at-signs’”

Even spelling out the sound a letter makes can assuage much head-scratching:

S’s becomes “Esses”
I’s becomes “Eyes”

Admittedly, sounds only makes sense in a sentence that has other spelled-out sounds:

There can be no eyes in the document.
versus
There can be no esses, eyes, elles or dees in the document.

Spelling out sounds is a matter of informal style. It is rarely done in professional writing.

Pluralising dates

This is typically done when referring to years (1970’s, 1800’s). The apostrophe can be left out as the distinction between number and letter is clear:

1970s
1800s

When referring to dates in word form, as in the nickname of an era or the general age of a person, the plural form of the word is used:

The roaring sixties
In his early thirties

Using the singular form with an apostrophe creates problems:

The roaring sixty’s
This implies that the number 60 both roars and possesses items.

Contractions

Apostrophes are used in contractions to show where a letter has been left out:

“Did not” becomes “Didn’t”
“Does not” becomes “Doesn’t”

Sometimes though the influence of popular language an apostrophe becomes left out, as in the much maligned little:

“Little” should become “li’l’”, but thanks to popular usage it is now “li’l”.

Referring to whole words

When referring to whole words, the placement of an apostrophe can change the meaning. For example:

No ands, ifs or buts. (there should be no conditions)
versus
No and’s, if’s or but’s. (those words should not appear)

When referring to the word itself, use apostrophes. If referring to the meaning of the word, use no apostrophes.