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表示与将来事实相反的假设，对将来表示怀疑，或将来的动作不太可能实现时，条件状语从句和主句的谓语动词有以下形式：（见表）
条件状语从句的动词形式(三种)
主句的动词形式
(1) If + 主语 + 动词过去式 (be的过去式用were). 通常要与一个表示将来的时间状语连用. (2)If+主语(任何人称和数)+should+动词原形
(3)if+主语(任何人称和数)+were to+动词原形
I (we)should+动词原形
主语+would (might, could)+动词原形
例句：
·If it rained tomorrow, our picnic wouldbe put off.
·If it should rain tomorrow, our picnicwould be put off.
·If it were to rain tomorrow, our picnicwould be put off.
·If you should fail, what would you do?
·If I were to see her tomorrow, I wouldtell her the truth.
·If it were to rain tomorrow, the meetingwould be put off.
·I bet Brazil will win the World Cup. Ifit should lose, I would treat you a meal.
注意：当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时，动词的形式要根据它所表示的时间来调整。
If you had worked hard, you would be very tired. (从句说的是过去，主句指的是现在。)
If he had followed the doctor’s advice, he would bequite all right now.
If I were you, I would have gone home.
Ifshe hadn’t trained so hard, she wouldn’t be able to run so fast.
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