Have you ever heard of the comma splice? Now you have! I was working with a student the other day who had written lots of sentences like this:

The number of charity shops in the UK has been rising steadily over the past few years, you can now see charity shops everywhere you look in city centres.

This punctuation is incorrect! You can’t just take two short sentences and put a comma between them! The example above is what is called THE COMMA SPLICE!

For the sentence above you could join them together with a conjunction:

The number of charity shops in the UK has been rising steadily over the past few years and you can now see charity shops everywhere you look in city centres.

Or you could use a semicolon when the parts of the sentence are very closely related:

The number of charity shops in the UK has been rising steadily over the past few years; you can now see charity shops everywhere you look in city centres.

Or you could separate them into two sentences:

The number of charity shops in the UK has been rising steadily over the past few years. You can now see charity shops everywhere you look in city centres.

Take a look at the OWL’s powerpoint on the use of commas. It makes it clear and simple and you can find it here.