Possessives, Contractions, and the Apostrophe
The most ignored punctuation mark, yet one that causes many writers grief and professors' headaches, is the simple apostrophe. The mark signals either a contraction or a possessive word (Can't, John's). Here are spots where writers trip up:
Unneeded and missing apostrophes:
Words that end in S
Contractions and "It": a special case
This situation is irregular. For "it is," use "it's." For the possessive, use "its" without the apostrophe.
By the way, many faculty members consider contractions too informal for academic writing. Thus "cannot" would be safe usage if a professor might mark "can't." Ask the faculty member to be certain of his or her policies in this case.
Some plurals and abbreviations are downright odd
Sometimes writers will include apostrophes for plurals such as DVD's, 1960's, and so on. The grammar texts cannot agree on this. It may be best to omit apostrophes in such cases for the sake of simplicity.
It is acceptable usage to use an apostrophe before a shortened term or date, such as "my '67 GTO raced his '69 'Cuda the other night."