My DS (age 10) and I have been arguingabout discussing when you use colons/semi-colons. So, he had in his story 'When he saw who it was, he was surprised. Sitting on the horse was a woman.' but he wanted to make it into a complex sentence.
He thought it needed a semi-colon, so making 'When he saw who it was, he was surprised; sitting on the horse was a woman.'
I thought it needed a colon so 'When he saw who it was, he was surprised: sitting on the horse was a woman.'
You can use either in this situation. I'd choose to go for a semi-colon as the two sentences are closely related, but still independent. It's not wrong to use a colon though. If you use the colon, you're explicitly subordinating the explanation (the bit after the colon) from the situation described (before the colon). If you use a semi-colon you're giving them more equal weight, but showing that they are related. A full stop would also be correct.
I always find it a bit of a problem to explain a semi-colon as a 'strong comma', ad this leads to the (ab)use of semi-colons where the second clause does not constitute an independent clause (I see this a lot in essays). Commas and semi-colons are not interchangeable; semi-colons and full stops are. For this reason, I'd prefer to teach them as a 'weak full stop'.
Thank you all so much. I know DS will be pleased to be right - but at least I can say I wasn't completely wrong!
Thing is, when I wrote my post, I realised I was not really correct as I wanted to write 'He wanted a semi-colon, thus: "When he saw . . . "' - but I needed you all to show me what what was what. It's not very clearcut though is it?
Grimma - in the context of his story a gnome certainly would have surprised him!