Perfect apostrophe control

Remember Sister Mary Loyola? she of the clothespeg strategy

Despite this early unpleasantness between us, we came to be great friends in the classroom. I was lucky enough to have her teach me a little later in primary school.

Grammar must have been her true teaching passion - because she was able to give the whole class of 35 snotty little kids the ability to use our cute and curly friend, the APOSTROPHE, with complete confidence.

So here's when to use one, which like all rules begins simply, then adds a little complexity

1. When you want to shorten by joining words and skipping a letter
   do not becomes don't
2. When you want to show possession
   Sarah's quilt

Fun facts for the possessive apostrophe:

1. Never with pronouns. Her's is not a thing. Ever. If Theresa has a pen, its hers
2. It moves if there are multiple owners of the possession.
   The cat's pyjamas - nightwear for one cat
   The cats' pyjamas - what all of the cats are wearing to bed

That's all. Quantities and plurals don't come into it. Thousand's isn't a thing either. Do the test: nothing shortened, nothing owned = no apostrophe.

Of course there is nothing rational about all this  - it's just how it is. I suspect there's an archaeology of grammar that could show how our use of the apostrophe is a remnant of a more wordy practice which has worn away in use.

Why does this matter? the main reason is clarity - we all want the ability to communicate what we mean to say. We all want to receive information that is unambiguous, easy to read. Some of us also want to give the impression of competence when we write, hoping this will lead to greater credibility and trust. The classic example:

Bless you Sister Loyola, wherever you are.