Although the apostrophe is often misunderstood and misused, the rules governing its proper usage are few, clear and simple. Primarily, apostrophes are used for two purposes: omissions of letters (such as contractions) and possessives. In a few specific and rare cases, the apostrophe is also used for plurals of single letters.
Use an apostrophe to represent the omission of a letter or letters. This rule most often applies to contractions, in which two words are combined into one. For example, the words "is" and "not" can be combined into "isn't"; in this contraction, the apostrophe replaces the letter "o" in the word "not."
The apostrophe clarifies the word and notifies readers that it was originally two words. Ignoring the rule and omitting the apostrophe would result in confusion. For example, if you leave out the apostrophe in "he'll" (he will), then a reader would appropriately assume that you wrote "hell," which has a completely different meaning.
Sometimes an apostrophe represents a missing letter (or letters) from one single word. This is especially useful for writing accented dialogue. Examples: "I'm draggin' my feet today." "My ol' dog has lived 15 years."
An apostrophe, along with an "s," can signify that one thing belongs to another. Example: "The cat's tail." The apostrophe and "s" communicate that the tail belongs to the cat. Without the apostrophe and "s," the phrase would simply read, "The cat tail," which is a plant, not an animal body part.
If the possessive noun is singular, then place the apostrophe before the "s." Examples: "The boy's red wagon creaked." "She ignored the stranger's greeting."
If the possessive noun is plural, and if the plural of that word is formed by adding "s" or "es" to the end of the word, then place the apostrophe after the "s." Examples: "The three bushes' branches were all broken." "The elephants' trunks splashed in the water."
If the possessive noun is plural but does not end in an "s," then treat it like a singular noun; add an apostrophe first, then an "s." Examples: "The people's wishes were ignored." "The men's voices echoed."
According to Strunk and White, if the possessive noun is singular but ends with the letter "s," you should still treat it like a normal singular possessive noun; add both an apostrophe and an "s." Examples: "James's essay was excellent." "The octopus's tentacles were long." Exception: The word "its" is possessive, but it does not take an apostrophe. Example: "The animal bared its teeth."
Plurals of Single Letters
An apostrophe can be used to indicate that a single-letter word is plural: for example, "She earned three A's last semester." According to the Owl at Purdue, however, some experts say that an apostrophe can only be used to pluralize single-letter words that are not capitalized. This is useful for some common expressions such as "cross your t's and dot your i's."
To determine whether an apostrophe is needed, ask these two questions.
Question 1: Are any letters missing? If there are, then insert an apostrophe in place of the absent letter(s).
Question 2: Is it possessive? If it is, then insert an apostrophe. To determine whether it is possessive, try this trick. If you can rearrange the phrase and add the word "of" to indicate the proper relationship (i.e., "the cat's tail" = "the tail of the cat"), then it is possessive; add an apostrophe.
Do not use an apostrophe to indicate a certain time period, i.e., "the 1990's." Instead, write, "I was born in the late 1970s."
Never use an apostrophe simply to indicate a plural--do not write, "The four pizza's were tasty," or "The employee's were diligent."
Only place an apostrophe in the word "it's" if you mean "it is." Otherwise, use "its." Examples: "The arrow missed its mark" (i.e., the mark of the arrow). "I think it's time to leave" (i.e., it is time to leave).