如题所述
1． 在look, seem 等系动词后引导表语从句。
如： She looks as if she were ten years younger． 她看起来好像年轻了十岁。 It seems as if our team is going to win． 看来我们队要胜了。
2． 引导方式状语从句。
如： She loves the boy as if she were his mother． 她爱这男孩，就好像她是他的母亲一样。 The child talked to us as if he were a grown up． 那孩子跟我们谈起话来，像个成年人似的
3.as if用于虚拟语气。
as if 从句用虚拟语气的情况。 当说话人认为句子所述的是不真实的或极少有可能发生或存在的情况时。从句虚拟语气动词时态的形式如下： （1）如果从句表示与现在事实相反，谓语动词用一般过去时。如： You look as if you didn’ t care． 你看上去好像并不在乎。 He talks as if he knew where she was． 他说话的样子，好像他知道她在哪里似的。 （2）从句表示与过去事实相反，谓语动词用“had＋过去分词”。如： He talks about Rome as if he had been there before． 他说起罗马来好像他以前去过罗马似的。 The girl listened as if she had been turned to stone． 那女孩倾听着，一动也不动，像已经变成了石头似的。 （3）从句表示与将来事实相反，谓语动词用“would／could／might＋动词原形”。如： He opened his mouth as if he would say something． 他张开嘴好像要说什么。 It looks as if it might snow． 看来好像要下雪了追答
if的用法如下
if
（表条件）如果；即使；是否；（表假设）假如
(引导条件从句) 如果
例：You can go if you want.
如果你想走就走吧。
(用于间接是非问句中) 是否
例：He asked if I had left with you, and I said no.
他问我是不是和你一起离开的，我说不是。
常与can、could、may、might等情态动词连用，表示婉转客气
例：If I could just make another small point about the weightlifters in the Olympics.
请允许我就奥运会的举重运动员们再稍说一句。
(用于从句句首或靠近句首的位置，表示礼貌的请求) 是否
例：I wonder if you'd be kind enough to give us some information, please?
不知能否请您向我们提供些信息？
(引导从句，提出次要的事实) 即使
例：If there was any disappointment it was probably temporary.
即使有任何遗憾，可能也是暂时的。
(尤指不能实现的愿望) 要是…多好
例：If only you had told me that some time ago.
要是你之前告诉我那件事该多好。
(表示判断) 好像
例：It looked as if she had forgotten how to breathe.
她看上去好像是忘了如何呼吸似的。
[表示条件或假设]如果；假如，倘若，要是：
例：If I knew, I would say.
如果我知道，我就说了。
例：If I were you, I would have failed.
如果我是你，我可能会失败的。
尽管；虽然；即使：
例：If he is little, he is strong.
他尽管年纪小，力气倒蛮大的。
例：If I am wrong, you are not right.
即使我错了，你也不对。
