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as if 用法归纳：
as if一般可由as though来代替。可在look, seem等系动词后引导表语从句。如:
It seems as if it is going to rain.
看来好像要下雨了。
as if 从句的语气及时态
当说话者认为句子所述的是真实的`或极有可能发生或存在的事实时,as if从句用陈述语气。如:
It sounds as if it is raining.
听起来像在下雨。