When to use an apostrophe. Lesson 101. "you are" = "you're", not "your" The apostrophe is used to denote a missing letter. In the case above it's the "a" that is missing, so the apostrophe takes it's place. If the English language must be destroyed on the net, perhaps "ur" can be used....shudder ! Kind regards, Henry Higgins p.s. Go Federer.