The apostrophe may be the most misunderstood punctuation mark in English. There are even websites dedicated to cataloging its misuse. Most punctuation marks fall between words to separate ideas or grammatical clauses, but the apostrophe is used within words and to combine multiple words which befuddles even native speakers.
This small mark has two primary uses: to signify either possession or omitted letters. Rather than say “the friend of Sam,” one can say “Sam’s friend” by adding a ‘s to the possessor (in this case, Sam). As for the second use, some common English words can be combined into a contraction, such as isn’t, don’t, and you’re. We often elide sounds and letters when speaking for the sake of convenience, and the apostrophe helps written language reflect its spoken equivalent. The word apostrophe comes from the Greek word apóstrophos which refers to a mark used in Greek to signify an omitted letter. It literally means a “mark of turning away.”
The apostrophe causes so much strife in part because it’s the culprit in two of the most commonly confused pairs in English: you’re/your and it’s/its. Possessive pronouns (like your and its) never take apostrophes, but their soundalike friends are contractions that require apostrophes. We all struggle with these when writing and proofreading our work, but here’s a trick: try replacing the ‘re or ‘s with are or is. If the syntax works, then you need the apostrophe; if not, it’s the possessive pronoun. For example, “It’s five o’clock” can be written “It is five o’clock,” while “The school locked its doors” cannot be written “The school locked it is doors.” Likewise, “You’re late” can be written “You are late,” while “I saw your note” cannot be written “I saw you are note.”