Fromkin & Rodman p. 31, number 9:
State some rule of grammar that you have learned is the correct way to say something, but that you do not generally use in speaking. For example, you may have heard that it's me is incorrect and that the correct form is It's I. nevertheless you always use me in such sentences; your friends do also, and in fact, I's I sounds odd to you.
Write a short essay presenting arguments against someone who tells you that you are wrong. Discuss how this disagreement demonstrates the difference between descriptive and prescriptive grammars.
We will make this assignment symmetrical. If you prefer, you can take the prescriptive side, by specifying a grammatical principle that you (believe that) you follow in your own speech and writing, and which bothers you to hear or read violated in the productions of others. If you take this side, please try to diagnose your feelings accurately: is the usage that annoys you genuinely mistaken, incoherent or degraded, or is it just different from what you expect, or associated with a kind of people that you don't like?
If you have trouble thinking of interesting cases, you can consult a prescriptive text for lists of prescriptions to react to. An excellent on-line resource is Jack Lynch's Grammar and Style Notes. Dr. Lynch does not always side with the traditional prescriptions (as for instance in the case of split infinitives), and you too should feel free to take either side.