Answer:
1. It's been years since I've enjoyed myself this much.
2. There is no point in trying to get Tim to lend you his car; you won't succeed.
3. If you aren't careful, you may fall.
4. He is so forgetful; he keeps forgetting his homework.
Explanation: I rearranged the sentences, although I added a word to the first one to make it more grammatically correct and flow better. Basically, all you have to do is flip the sentences around. I hope it's alright that I used semi-colons as well; they just make some sentences sound, well... better. If not, here are alternate versions of the questions I used them in, but without semi-colons:
2. There is no point in trying to get Tim to lend you his car because you won't ever succeed.
4. He is so forgetful as he keeps forgetting his homework.