Apostrophes and how to cope with them

http://www.dictionary.com/e/apostrophes-101/

“The apostrophe causes so much strife in part because it’s the culprit in two of the most commonly confused pairs in English: you’re/your and it’s/its. Possessive pronouns (like your and its) never take apostrophes, but their soundalike friends are contractions that require apostrophes. We all struggle with these when writing and proofreading our work, but here’s a quick trick: try replacing the ’re or ’s with are or is. If the syntax works, then you need the apostrophe; if not, it’s a possessive pronoun. For example, “It’s Sunday” can be written “It is Sunday,” while “The school locked its doors” can’t be written “The school locked it is doors.” Likewise, “You’re late” can be written “You are late,” while “I saw your note” cannot be written “I saw you are note.”

The apostrophe has a number of other lesser-known uses. It can replace omitted numbers (e.g. the class of ’72, the ’20s, etc.) and letters in written pronunciation slang e.g. gone fishin’. It can also be used to indicate plural letters, as in p’s and q’s, two A’s and four B’s, etc.”