第三层次：
省略If的条件句 (Without/But for)
1) Without water, we would die.
—If we had no water, we would die.
2) Without your help, I would have failed in the exam yesterday.
—If you hadn’t helped me, I would have failed in the exam yesterday.
第四层次：
混合虚拟：
1) If I had studied hard before, I would get the good marks now.
2) If I had drunk enough water just now, I would not be thirsty now.
第五层次：
半“虚”半不“虚”：
前“虚”后不“虚”。
题一：
1. I would have attended the party yesterday, but I ____ ill.
A. had fallen B. fell C. would fall D. have fallen
2. I would lose weight, but I ____ a lot now.
A. eat B. ate C. had eaten D. would have eaten
