Relative clauses, and some good sites on the web
We have gone through the relative clause, both the defining and the non-defining one. Now you can do some more reading up on the subject, and/or do some exercises.
English grammar online
British Council
A guide to learning English
It might be good with a more fun explanation to "who or whom"
Oatmeal
Here is another link which might be of use to you, one about the semicolon
And here you can find the whole Advance Grammar in Use. Work with whatever chapter you think you need. There is something for everyone. You need to print the pages you want - do not print the whole thing!