Like the comma, the apostrophe has conventions that can be boiled down to a small list.
The apostrophe is used to indicate a noun has ownership. You do this by adding ‘s.
• If the noun is plural and ends in -s, you would add only an apostrophe without the “s” as in, “The boys’ hats were identical.”
• If there is dual possession, use the apostrophe with the last noun only as in, “I went to visit John and Mary’s new baby.” But if the possession is on two levels, you will use an apostrophe for both such as “I went to see John’s and Mary’s new babies.”
• If the noun is compound, use the apostrophe in the last element as in, “My mother-in-law’s behavior has become very strange.”
• You will also use an apostrophe to show possession with an indefinite pronoun such as anyone, someone or something as in, “This season would exhaust anyone’s patience.”
Exception: Some possessive personal pronouns such as your, yours, hers, its, ours, their, and theirs do not take apostrophes, such as, “The dog chased its tail, making us laugh.”
Use an apostrophe to mark contractions. In that case, the apostrophe substitutes for the missing letter or letters. It’s stands for “it is” and haven’t stands for “have not.”
use an apostrophe to pluralized certain numbers, letters and abbreviations.
• The Olympic contender scored a series of 10’s.
• Mary’s report card had all A’s.
• The teenagers were asked for their ID’s.
Like with most punctuation, the conventions are few and easy to memorize.
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