Apostrophes serve two functions in English: showing possession (John's book) and marking missing letters in contractions (it's = it is). Misusing them — especially the infamous "it's vs its" confusion — is one of the most noticed written errors in English.
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Apostrophes for Possession
Add apostrophe + s ('s) to show that something belongs to a person or thing: "the manager's decision", "the dog's lead". For plural nouns ending in -s, add only the apostrophe: "the students' results", "the managers' meeting". For irregular plurals (children, men, women), add 's: "the children's toys". Proper nouns ending in -s can take either form: "James's car" or "James' car" — both are acceptable.
Apostrophes in Contractions
Apostrophes replace missing letters in contractions. Common examples: I'm (I am), it's (it is/has), he's (he is/has), she'd (she would/had), they're (they are), don't (do not), won't (will not), can't (cannot), isn't (is not), haven't (have not). The apostrophe is placed exactly where the missing letters are.
Its vs. It's — The Most Common Confusion
"It's" = it is or it has. "It's cold today." "It's been a long day." "Its" (no apostrophe) = possessive pronoun, like his or her. "The dog wagged its tail." A useful test: substitute "it is" or "it has". If the sentence still makes sense, use "it's". If not, use "its". Never write "it's" when you mean the possessive.