Apostrophes are punctuation marks that are used for a variety of purposes. Two of the main uses of apostrophes are for forming possessives and for showing where letters have been omitted in words.
Apostrophes in Possessives:
When making a noun possessive, you typically add an apostrophe + s. When the noun is plural and already ends in "s," just add the apostrophe after "s". If the noun is singular and ends in "s," you add the apostrophe + s if it's one syllable. You add just the apostrophe after the "s" if it is more than one syllable.
Apostrophes for Omitted Letters
When making a contraction, an apostrophe is used to show where letters have been omitted. There are also other words that we don't think of as contractions that are formed with an apostrophe to show where letters were omitted.
Examples of Apostrophes with Possessives
dog's bone (singular)
girl's bag (singular)
girls' locker room (plural)
students' shoes (plural)
Mark's house (singular)
Mrs. Sims's classroom (singular, ends in "s," one syllable)
Mr. Jones' car (singular, ends in "s," two syllables)
Children's playground (plural but does not end in "s")
Examples of Apostrophes for Omitted Letters
Cannot = can't
Will not = won't
Is not = isn't
It is = it's
o'clock (of the clock)
'tis (it is)
'twas (it was)