注意:主句中的should通常用于第一人称,would可用于任何人称,同时也可根据意思用情态动词could, might等代替should, would。

　　1)表示与现在事实相反:

　　If I were you, I should stay at home.

　　If the peasants could farm the land themselves, food production would be much higher.

　　If fewer cash crops were grown, more food could be produced and there would be less or no starvation.

　　2)表示与过去事实相反:

　　If you had come yesterday, you would have met him.

　　If the hurricane had happened during the daytime, there would have been many deaths.

　　3)表示与将来事实相反:If you came tomorrow, we would have the meeting.

　　If it were to/should rain tomorrow, the meeting would be put off.

　　If it should rain, the crops would be saved.

　　4)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。例如:If you had followed the doctor's advice, you would be quite all right now. (从句说的是过去,主句指的是现在.)

　　5)以上句型可以转换成下列形式:①从句省略if,用倒装句式"were, had, should+主语"。例如:Were I in school again, I would work harder. Had you been here earlier, you would have seen him. Should there be a meeting tomorrow, I would come.②用介词短语代替条件状语从句。例如:Without air, there would be no living things. I would not have succeeded but for your help.