The Underground Grammarian Strikes Again
Besides making another post about the "liberal moderation versus extremism" argument that he began, Kevin Drum also has these handy tips for writers with homonym trouble. I've never had that myself, the ointment's always worked quite well. In any case, in explaining the usage rule for "its versus it's" he slips up a bit, touching on the very reason many people still have apostrophe problems with that three-letter word. Quoth Kevin, "It's" always means "it is" (or "it was"). Use "its" in all other cases. Would that it were that easy. Alas, "it's" can also be used to mean "it has" (for instance, It's been a hard day's night...), and that's where I think a lot of people trip up and use an apostrophe in the possessive form. Because "to have" is to possess, see? Even though "has" in this particular case is actually part of a present perfect (progressive) verb, not a possessive verb at all. I only mention this because "it's versus its" has always been one of my pet peeves as well, so I've thought a lot about why it confuses people. My rule on this is, if you can mentally substitute "his" or "hers" or "theirs" for "its" and the sentence still sounds right, that means the word is a possessive and does not take an apostrophe. Use "it's" in all other cases.