One of the obvious failures in the teaching of English grammar involves the apostrophe, and as a result vast numbers of people are blithely unaware of its proper usage; they include or exclude it at will, never being quite confident. As is so often the case the rules are simple, and, properly used, the apostrophe serves a very important purpose.
There are three circumstances where it should be employed:
to indicate that a noun is possessive: “It is John’s hat”;
to indicate a verb-phrase contraction: “Aren’t you joining us?”; and
to denote a deletion of letters or numbers in a word: “burbs” (for suburbs) or “68′” (for 1968). (Note: An apostrophe should not be used where a word is abbreviated properly, such as “Penna.”)
We’ll be talking more about each of these uses of the apostrophe. In the meantime think a bit before using or omitting it.
– Ken Butera