Ответ:
1. If the weather is fine, we will go for a picnic.
2. If you will go out in cold weather without a coat, you would catch a cold.
3. If it rains this afternoon, I will take my umbrella.
4. If you are well and exercise regularly, you will live 100 years.
5. You would have had a stomach ache if you ate too much of that cake.
6. If you did not drive carefully, you will have an accident.
9. If you sent the letter by first class mail, it will get there the following day.
10. If I saw a cheap used car, I would buy.