1. CONJYou use
if in conditional sentences to introduce the circumstances in which an
event or situation might happen, might be happening, or might have happened.
(引导条件从句) 如果
例：She gets very upset if I exclude her from anything.
如果有什么事我将她排除在外，她就会非常难过。
例：You can go if you want.如果你想走就走吧。
2. CONJYou use
if in indirect questions where the answer is either "yes" or "no."
(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。
3. CONJYou use
if to suggest that something might be slightly different from what you
are stating in the main part of the sentence, for example, that there might be
slightly more or less of a particular quality. (表示与主句所述事实可能有少许不同) 即使
例：Sometimes that standard is quite difficult, if not impossible, to achieve.
有时那个标准，即使并非不可能，也是很难达到的。
4. CONJYou use
if, usually with "can," "could," "may," or "might," in a conversation
when you are politely trying to make a point, change the subject, or interrupt
another speaker. 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。
5. CONJYou use
if at or near the beginning of a clause when politely asking someone to
do something. (用于从句句首或靠近句首的位置，表示礼貌的请求) 是否[礼貌]
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？
6. PHRASEYou
use if not in front of a word or phrase to indicate that your statement
does not apply to that word or phrase, but to something closely related to it
that you also mention. 即便不
例：She understood his meaning, if not his words, and took his advice.
她即便不懂他的语言，也明白了他的意思，并且接受了他的建议。
7. CONJYou use if to introduce a subordinate clause in which you
admit a fact that you regard as less important than the statement in the main
clause. (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。
8. PHRASEYou
use if ever with past tenses when you are introducing a description of a
person or thing, to emphasize how appropriate it is. (与过去式连用，强调对人或事物的描述极为恰当) 如果有…的话[强调]
例：I became a distraught, worried mother, a useless role if ever there was one.
如果有个心烦意乱、忧心忡忡的母亲，一个完全无用的角色，那就是我。
9. PHRASEYou
use if only with past tenses to introduce what you think is a fairly good
reason for doing something, although you realize it may not be a very good one.
即使仅仅 (与过去式连用，引出能勉强说明做某事的原因)
例：She always writes me once a month, if only to scold me because I haven't answered her last letter yet.
她总是一个月写一封信给我，即使仅仅是为了责备我还没有回她的上一封信。
10. PHRASEYou
use if only to express a wish or desire, especially one that cannot be
fulfilled. (尤指不能实现的愿望) 要是…多好[情感]
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。
11. PHRASEYou use as if when you are making a judgment about something
that you see or notice. Your belief or impression might be correct, or it might
be wrong. (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。
12. PHRASEYou use as if to describe something or someone by comparing
them with another thing or person. (表示类比) 仿佛
例：He points two fingers at his head, as if he were holding a gun.
他用两个手指对着自己的脑袋，仿佛正拿着一把枪。
13. PHRASEYou use as if to emphasize that something is not true.
(强调某事不是真的) 好像…一样[口语]
例：Getting my work done! My God! As if it mattered.
把我的活儿干完！我的天哪！好像这很要紧似的。
as if 从句的作用
1．引导表语从句， 常与as,look, seem,feel,taste,smell,sound等系动词连用。
如：She looks as if she were ten years younger．
她看起来好像年轻了十岁。
2. 引导方式状语从句，表示“事情是怎么做的”或“一个人或事物是什么样的”。
如：
如：She loves the boy as if she were his mother．
她爱这男孩，就好像她是他的妈妈一样。
3.as if 还可用于省略句中，如果as if 引导的从句是“主语+系动词”结构，可省略主语和系动词，这样as if 后就只剩下名词、不定式、形容词（短语）、介词短语或分词。
如：He acts as if （he were） a fool．
他做事像个傻子。
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