By Leigh Anderson on LifeHacker site:
For some of us, grade-school grammar lessons haven’t stuck. I managed fairly well with my trusty Elements of Style until pretty recently, but the ongoing assault on grammar that is the Internet occasionally shakes my confidence. (Every time I want to type rein, rain, or reign, for example, I need to close my eyes and meditate for a moment.)
If you’re one of those people for whom grammar and punctuation don’t come easily (or aren’t a native English speaker), this handy flowchart from GrammarCheck.net will come in handy: It will help you navigate all the questions that determine whether you need an apostrophe or not, from the big (Are you making something possessive?) to the small (Is it a single-letter word?).