Ah, the apostrophe, the single most used and abused punctuation mark in the history of the latin alphabet. Perhaps people get confused with its use because in general terms (we will get on to its use as a diacritic mark later) it has not one but two. Firstly, it is used when one (or more) letters are omitted, as in didn't for did not. Next, it is also used to mark a possessive - such as the bee's knees.
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.
As for the use of the apostrophe as a diacritic, that is when a glyph is added to a letter (sometimes above, sometimes below, sometimes within or even between two letters - grief). It is used to modify how the letter is pronounced. A diacritic mark, in some of its forms is also known as an accent. Just thought we would confuse the issue a little.
