With examples from Star Trek and Psych.
As a punctuation mark in English, the apostrophe has two major uses:
1.For the possessive forms of nouns.
2.To indicate omission of a letter.
Apostrophes are used when you want to indicate ownership. For singular nouns, at the end of the word put an apostrophe followed by “s” to show possession:
The shuttle's proximity alert blared as Kirk set course for the Klingon warbird.
If you have two or more nouns that are jointly possessive, add an apostrophe to the last noun only:
Spock and Uhura's favorite place to eat lunch together was on the observation deck.
But if you have two or more nouns in a sentence that are separately possessive, add an apostrophe to all the possessive nouns:
Sulu's and Chekov's chairs were covered in some kind of pink slime.
For plural nouns ending in “s,” just place an apostrophe after the “s” indicating the plural. Leave off the extra “s”:
Bones woke with a pounding headache, the sound of the natives' drums still thrumming in his blood.
If the plural form of the noun is irregular and doesn't end in “s,” add an apostrophe and then “s” just like for singular nouns:
Gus was never able to figure out how Shawn managed to infiltrate the Santa Barbara Women's Dance Club.
But what if you have a singular noun or a name that ends in “s”? Should the second “s” be dropped or not?
Shawn wondered what Gus' car would look like painted orange.
Shawn wondered what Gus's car would look like painted orange.
Both examples above are considered correct. Different style guides adhere to different conventions, so if you're following a specific style guide make sure to check what it says on the matter. If not, go with whichever form you like best. Just remember to stay consistent within your writing!
For more information about possessives and sibilants, see this earlier post.
Possessive pronouns, such as "his," "hers," and "its" are an exception to the above guidelines. Possessive pronouns do not need apostrophes.
Let's take a closer look at "its," because writers often confuse "its" with "it's," when in fact they cannot be used interchangeably:
Kirk wrestled with the lizard creature, trying to avoid its sharp fangs.
As noted above, "it" as a possessive pronoun never takes an apostrophe. The only time “it” needs an apostrophe is when it is a contraction:
“Why do you always get to do the wrap-up, Shawn?” asked Gus. “It's not fair.”
It's easy to know whether your “it” needs an apostrophe or not: if you can replace “it's” with “it is” and the sentence still makes sense, then you need the apostrophe. If not, then leave the apostrophe out!
Never use an apostrophe to indicate a plural noun! This is a common mistake that many writers (not many writer's) make.
Well, never use an apostrophe to indicate plurals except in the following cases (come on, you didn't think it would be that straightforward, did you?):
1. Use apostrophes for plurals of letters: K's, G's, S's, etc.
2. Use apostrophes with plurals of numbers to avoid ambiguity: 1's instead of 1s, since the latter could be mistaken for the word “Is.”
If you're unsure whether your noun should be possessive or plural, ask yourself what your noun is possessing.
The Andorian Ambassador was scheduled to arrive in one week's time.
What is the word “week” possessing? It's possessing the word “time.” To double check, reword the sentence to eliminate the apostrophe:
The Andorian Ambassador was scheduled to arrive in the time of one week.
Now let's compare that to the following example:
In only a matter of hours the ship was overrun with tribble's.
The ship was overrun with the tribble's what? The word “tribble” isn't possessing anything in that sentence, so it's a plural, not a possessive. There should be no apostrophe.
In only a matter of hours the ship was overrun with tribbles.
The second major usage of the apostrophe in English is to indicate omission of single letters; in other words, to form contractions. A contraction is an informal construction that joins together two words, dropping one or more letters from the original words. In contractions, apostrophes are put in place of those dropped letters.
Lassiter couldn't begin to explain how Shawn had known about the cigar.
In the above example, “couldn't” is a contraction of “could not,” but the “o” from “not” has been dropped and replaced with an apostrophe. This is also a good way to check whether you've placed the apostrophe in the correct place in a contraction:
Lassiter could'nt begin to explain how Shawn had known about the cigar.
Here the apostrophe is in the wrong position because it's not taking the place of any letters.
And that should give you the basics on when and how to use an apostrophe!