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used to say that one thing can, will or might happen or be true, depending on another thing happening or being true 如果；假若；倘若 If you see him, give him this note. 你要是见到他，就把这个便条给他。 I'll only stay if you offer me more money. 你给我更多的钱我才会留下。 If necessary I can come at once. 如果有必要，我可以马上来。 You can stay for the weekend if you like. 你如果愿意就留在这里过周末吧。 If anyone calls, tell them I'm not at home. 要是有人打电话来，就说我不在家。 If he improved his IT skills, he'd (= he would) easily get a job. 他如果提高了自己的信息技术技能，就会容易找到工作。 You would know what was going on if you'd (= you had) listened. 你若是注意听了就会知道发生什么事了。 They would have been here by now if they'd caught the early train. 假若他们赶上了早班火车，现在就该到这里了。 If I was in charge, I'd do things differently. 假若由我负责，我不会这样办事的。 - (rather formal)
If I were in charge… 假若由我负责…
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when; whenever; every time 当；无论何时；每次 -
used after ask, know, find out, wonder, etc. to introduce one of two or more possibilities （用于 ask、know、find out、wonder 等之后，引出两个或以上的可能性之一）是否 Do you know if he's married? 你知道他是否结婚了？ I wonder if I should wear a coat or not. 我不知道该不该穿外套。 He couldn't tell if she was laughing or crying. 他弄不清楚她是在笑还是在哭。 Listen to the tune and see if you can remember the words. 听听曲调，看你能否记起歌词。
Grammar Point if / whetherif / whether- Both if and whether are used in reporting questions which expect ‘yes’ or ‘no’ as the answer:
if 和 whether 均用于要求以 yes 或 no 作答的转述疑问句中： She asked if/whether I wanted a drink., 她问我要不要喝点什么。
不过 whether 与 discuss、consider 和 decide 等动词连用听起来更自然。提供选择时用 if 或 whether 均可： We didn’t know if/whether we should write or phone. 我们不知道是写信好还是打电话好。
在最后这一句型中，一般认为 whether 较正式，更适用于书面语。
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used after verbs or adjectives expressing feelings （用于表示情感的动词或形容词之后） -
used to admit that something is possible, but to say that it is not very important （承认某事可能，但不很重要）即使，虽然 -
used to ask somebody to listen to your opinion （请对方听自己的意见） If you ask me, she's too scared to do it. 依我看，她被吓得不敢做了。 If you think about it, those children must be at school by now. 想想吧，那些孩子现在一定在学校了。 If you remember, Mary was always fond of animals. 记得吧，玛丽总是喜欢动物。
used before an adjective to introduce a contrast （置于形容词之前，引出对比）虽然，尽管 He's a good driver, if a little over-confident. 他是个好司机，虽然有点过于自信。 We'll only do it once—if at all. 我们就是干也只会干一次。
- (formal)
used with will or would to ask somebody politely to do something （与 will 或 would 连用，表示客气地请求） used before could, may or might to suggest something or to interrupt somebody politely （置于 could、may 或 might 之前，提出建议或客气地打断别人的话）
Word OriginOld English gif, of Germanic origin; related to Dutch of and German ob.
Idioms
even if
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despite the possibility that; no matter whether 如果： 尽管有这种可能性；不管是否I'll get there, even if I have to walk. 我就是走也要走到那儿去。
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despite the fact that; even though 如果： 尽管事实；即使He is a great leader, even if he has many enemies. 他是一个伟大的领袖，即使他有很多敌人。
if and when
if anything
used to express an opinion about something, or after a negative statement to suggest that the opposite is true （表达看法，或用在否定句之后表示反面意见才对） I'd say he was more like his father, if anything. 依我看，如果一定要说他像谁的话，他比较像他的父亲。 She's not thin—if anything she's on the plump side. 她并不瘦，其实她还有点胖呢。
if I were you
used to give somebody advice （提出劝告时说）
if not
used to introduce a different suggestion, after a sentence with if （用在 if 引导的句子之后）不然，要不 used after a yes/no question to say what will or should happen if the answer is ‘no’ （用于 yes／no 疑问句之后，表示如果答案是 no，将会或应该发生什么） used to suggest that something may be even larger, more important, etc. than was first stated （表示可能更大或更重要等）
if only
if so
if that is the case 但愿 Do you do research in your studies? If so, what research methods do you use? 你在学习中做研究吗？如果有，你用什么研究方法？
it’s not as if
only if