Who's vs. WhoseHow to use "who's" and "whose" correctly
Who’s is a contraction for “who is” or “who has.” It is never a possessive pronoun. Possessive pronouns in general don’t use apostrophes: see his, hers, theirs, its (see also the Its vs. It’s entry), etc.
Whose going on the trip? (Does going belong to someone? Obviously that doesn’t make sense.)
Who’s going on the trip? = Who is going on the trip?
Whose been there before? (Again, there’s nothing that belongs to someone here.)
Who’s been there before? = Who has been there before?
Who’s suitcase is this? (“Who is suitcase is this?” and “Who has suitcase is this?” don’t make sense.)
Whose suitcase is this? (The suitcase belongs to someone.)
If you’re unsure about which one is the right one to use in a given sentence, try substituting who is and who has for who’s and then asking yourself whether the sentence makes sense or whether there’s something that belongs to someone.
Another source of confusion regarding these words is the false impression that the pronoun whose can only be used to refer to people. In fact, whose can be used to refer to any noun, because there is no other word in English that has the same function as whose; there is no equivalent possessive form of that or which.
the mysterious book that its cover was all black (This construction is grammatically incorrect.) the mysterious book thats cover was all black (No such word exists, and that’s would mean “that is” or “that has.”)
the mysterious book whose cover was all black = the book’s cover was all black
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage