Ok, so perhaps I’m a little overly sensitive on this one, but apostrophe mis-use really drives me bonkers! (yes, yes, it’s a short drive, thank you).
I can’t believe that people just don’t get the very simple rules on when to use and when NOT to use an apostrophe. It seems that the great unwashed seem to feel that it’s seasoning which can be tossed in whenever it looks neat, and that it’s how you make something plural. (groan)
Here’s a really great page which spells it out in graphic detail: http://theoatmeal.com/comics/apostrophe
However, in a nutshell: An apostrophe does not make something plural. Writing “we have 4 unit’s in our apartment building” is just plain wrong. Yet, you’ll see it all over the web, and all over storefront advertising.
An apostrophe basically has two uses; showing possession, or replacing missing letters in a contraction.
An example of possession would be: “I like Joe’s car” (which could be rewritten: I like the car belonging to Joe). You can use this rewriting trick to see if an apostrophe is appropriate. In our example about apartment units above, you can’t rewrite this way because since “unit” doesn’t possess anything, it wouldn’t make sense. So, that’s how you know not to use an apostrophe in that case.
An example of a contraction would be “I don’t understand“. “Don’t” is a contraction of “Do Not”. The apostrophe takes the place of the missing “o”. In our apartment example above, there are clearly no missing letters, so again, the apostrophe does not belong. Poor apostrophe. We can make it up to him by correctly using him in “it’s fun to get it right“. You can rewrite that one with the missing letter put back in: “it is fun to get it right“. That makes perfect sense, so it’s okay to use an apostrophe. (Ooo… was that one correct? Try to rewrite it and see)