grammar test from Bristol University

Here is a grammar link. Test your knowledge of who and whom.
In this exercise, you will be presented with some sentences with a word missing. You will have to decide whether the missing word should be 'who' or 'whom'. Remember, if you can re-arrange the sentence and put a subject pronoun (I or he) in the space, you should use 'who'. If you can put an object pronoun (me, him) in the space, you should use 'whom'.
So follow the link
and maybe do some more exercises.