Apostrophe catastrophe

For some time, I've been fretting quietly about the increasingly sloppy use of the apostrophe one sees all over the place. It was bad enough when it was confined to market stallholders (a sighting of the sign "cres's 25p" was made by a Guardian reader once), but now it seems that grammatical ignorance has spread to the intellectual élite.

Before it goes too far, here are some simple rules which students untutored in these matters should commit to memory.

Plurals. When there is more than one of something, add an "s" (or "es" if the singular word ends in "s", "z" or "x"). Don't use an apostrophe. For example, "I have submitted no essays (not "essay's") this term." There are other irregular plurals for wolf (wolves), sheep (sheep) and so on — you just have to learn these.

Genitives. These are also known as possessives, and refer to the addition of " 's" to a word when it owns someone or something, unless the owner is a plural ending in "s" when you just add an apostrophe. The apostrophe always goes after the thing that does the owning. So, "the boy's books" refers to the books which belong to the boy, while "the boys' books" refers to the books which belong to the boys.

Third person possessive pronouns. You wouldn't write "hi's book" for the book belonging to him. Similarly, you should write "its (not "it's") book" for the book belonging to it. This is often confused with:

Contractions. "It is" can be contracted to "it's" in the same way at "don't" for "do not" and so on.

This may all seem a bit complicated for students not lucky enough to have had a Scottish school education, but it will help immensely if their writing does not appear illiterate, and therefore it's worth learning.

STOP PRESS: I have just personally encountered the most superb example of a misunderstanding of apostrophes that I have ever come across. Here's the sentence; see if you can spot what's going on in the author's mind:

"We should all be genius' by the end of the course!"

Firstly, they've made the mistake of thinking that the plural of genius should have an apostrophe in it, and the idea that popped into their brain was that it should therefore be "genius's".

But then they've taken the archaic rule that people who feature in the Bible whose names end in an "s" don't add a final "s" after the apostrophe when they own something (e.g., "Jesus' Blood Never Failed Me Yet", the title of Gavin Bryars's stunning minimalist piece for tramp and orchestra) and over-generalised it to cover the possessive of any word that ends in "s".

Here's another one I've spotted in a local shop: "We sha'll be closing at 5pm today". He'll's be'll's.

Wonderful! If you've got any good examples, please send them to me.

A READER WRITES: "Sarahs Sandwiche's" reads the sign on a cafe opposite Cambridge bas station, says Annette Jones. I can't look at it — it makes my blood boil every time.