8. as if / though引起的从句

当as if / though引起的从句所表达的内容完全与实际情况相反或者纯粹是一种假设时，通常要用虚拟语气。如果从句表示与现在的事实相反，谓语动词用一般过去时;如果从句表示与过去的事实相反，谓语动词用过去完成时;如果从句表示与将来的事实相反，谓语动词用would （might， could） + 动词原形;

They talked as if they had been friends for years.

I remember the whole thing as if it happened yesterday.

It looks as if it might rain.

Note： 如果as if / though引起的从句所表达的内容被看作是事实或者有可能是真实的，则要用陈述语气。

It seems as if it is going to rain.

The meat tastes as if it has already gone bad.

9. lest， for fear that和 in case引起的从句

当lest， for fear that和 in case表示“以免，以防，生怕，惟恐”等时，在它们引起的从句中，谓语动词常用should + 动词原形。

He ran away lest he should be seen.

He’s working hard for fear that he should fall behind.

He left early in case he should miss the last train.