一、基本用法

以as if (as though)引导的方式状语从句或表语从句，有时用虚拟语气。
  
   若表示与现在事实相反，谓语动词用一般过去时；

   若表示与过去事实相反，用过去完成时；

   表示将来的可能性不大，用would (might, could)+动词原形。
 

He acts as if he knew me. 他显得认识我似的。
 

They treat me as though I were a stranger. 他们待我如陌生人。
 

He talks as if he had been abroad. 他说起话来好像曾经出过国。

二、两点说明

1.从句所表示的内容若为事实或可能为事实，也可用陈述语气.
 

It looks as if we’ll be late. 我们似乎要迟到了。
 

This meat tastes as if it has already gone bad. 这肉吃起来似乎已经坏了。
 

在现代英语中，有时即使从句内容不是事实，也可能用陈述语气.
 

I feel as if [as though] I’m floating on air. 他感到好像漂浮在空中。
 

The stuffed dog barked as if it is [were, was] a real one. 玩具狗叫起来像真狗。
 

2.注意 It isn’t as if…的翻译：
 

It isn’t as if he were poor. 他不像穷的样子(或他又不穷)。
 

It isn’t as if you were going away for good. 又不是你离开不回来了。