虚拟条件句中的条件从句除用if引导外，还可用when, unless, lest, suppose, as if, for fear, in case, on condition 等词语来引导。
The peasants prepared to feed the city when it should be freed.
Lest you should not have heard all, I shall begin at the beginning.
Unless I were well, I wouldn’t be at school.
Suppose you were given a chance to study in America , would you accept?
假如给你一个到美国学习的机会，你会接受吗？（suppose 或 supposing =what if）
Supposing it were so？（=Supposing it were so，what would happen？）
He can use the bicycle on condition that he should return it tomorrow
In case I forget , please remind me of my promise
Susan is walking slowly ,as if she were tired.