Yet many people still make the fundamental error of using it to indicate plurals - such as I want to buy some cabbage's. This is often known as the greengrocer's apostrophe in the UK as you often see it on shop notices or even - horror of horrors - their very signs.
All is not lost, however. This song puts the rules across in a simple and very immediate way. Plus, to make it even cooler, the song has a very 80s synth thing going on.
However, enjoy the song - it's a lot of fun!