How do you use apostrophes?

The first thing to understand about using apostrophes is that they should never be used to form plural Words. This is grammatically incorrect, though unfortunately common. An apostrophe is commonly, but erroneously, used in numbers such as calendar decades or centuries instead of using the correct form, e.g. 1800s or the 1970s.

Apostrophes should only be used for possessive nouns (but not pronouns), e.g. the cat's food dish; Roger's BMW. They are not used in possessive pronouns, e.g. yours, ours, theirs, its.

They are also used for omissions in contractions, e.g. it's meaning it is (note the difference in how its is used in the previous sentence) or can't meaning cannot.

In some situations it is acceptable to use an apostrophe to clarify a plural. For example, "Make sure you dot all the i's and cross all the t's". This, however, is not an established rule.