Answer:
Use an apostrophe to indicate possession with singular nouns, except for nouns ending in "s" where you can use either an apostrophe alone or an apostrophe followed by an "s". For example: "the cat's toy" or "the cat's toys" or "James's book" or "James' book".
Use an apostrophe to indicate possession with plural nouns that do not end in "s", by adding an apostrophe and an "s". For example: "the children's toys".
Use an apostrophe after the "s" to indicate possession with plural nouns that already end in "s". For example: "the boys' toys".
Use an apostrophe to indicate contractions, such as "it's" for "it is" and "you're" for "you are".
Do not use an apostrophe to indicate plural nouns, except for certain cases such as letters, numbers, and abbreviations. For example, "mind your p's and q's" or "She got straight A's in her classes."