英语四、六级考试语法精要解析：虚拟语气8

　　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.