There are only two situations that call for apostrophes in English.
In a contraction, two words are joined to create one, and a few letters are left out. The apostrophe replaces the missing letters.
can + not = can’t
you + are = you’re
I + will = I’ll
Apostrophes combined with an s are also used to show possession, or ownership.
If the word does not end in an s, add apostrophe s to the end.
For example: professor’s lecture children's songsbook’s theme
If the word is plural and ends in s, just add the apostrophe.
For example: teachers’ lounge girls’ dorm roses’ stems
If the word is singular and ends in s, either is acceptable.
For example: James's hat or James' hat
Common apostrophe mistakes to watch out for
Maybe because apostrophes and the letter s are linked in many people’s minds, they sometimes add apostrophes when simply writing a plural word. This is a mistake. The s alone is enough to show pluralism; an apostrophe should only be used for possession or contractions.
incorrect The frame’s held picture’s of the family.
correct The frames held pictures of the family.
Possessive pronouns can cause confusion because they do not need apostrophes. They are possessive in meaning.
incorrect Monica repeated to Ross that the recipe was her’s. The bond of their’s will never be broken.
correct Monica repeated to Ross that the recipe was hers. The bond of theirs will never be broken.
**This is especially confusing for its/it’s. The possessive pronoun does not have an apostrophe; the contraction does.
correct It’s a beautiful day for a picnic
correct The dog chews on its bone.