Omission of lettersUse an apostrophe to signal the omission of a letter or series of letters in words of contraction.
did not = didn't
Forming pluralsUse an apostrophe to form the plural of letters, figures, or words referred to as words only if confusing or awkward plural formations would result. When no confusion would result, do not use the apostrophe.
He's a tough judge, so you'd better watch your p's and q's.
Omission of numeralsOccasionally, an apostrophe may be used to mark the omission of numerals when writing dates in situations involving names, titles, or slogans.
the Roaring '20s
Do not shorten a date when it is used simply to refer to a particular yearly date.
I attended Harmony College in 1994.