/ɪf; ɪf/
conj
1 on condition that; supposing 假如; 如果; 倘若; 要是.
(a) (used with the present and present perfect tenses for highly predictable situations 与现在时态和现在完成时态连用表示预料到的情况)
*I'll only stay if you offer me more money. 假若你肯多给钱, 我就留下.
* If you have finished eating you may leave the table. 你要是吃完了, 就可以离席了.
* If (it is) necessary I will come at 6. 如有必要, 我6点钟来.
* You can stay to dinner if you like. 你愿意的话, 可以留下一起吃饭.
* If anyone calls tell them I'm not at home. 有人来电话, 就说我不在家.
* (fml 文) If the patient should vomit, turn him over with his head to the side. 倘若病人要呕吐, 就帮他翻过身来, 头侧向一边.
(b) (used with a past tense for imaginary situations 与过去时态连用表示假想的情况)
*If you learned to type you would easily find a job. 假使你学过打字, 就容易找工作了.
* If he were here I could explain to him myself. 要是他在这里, 我就可以亲自向他解释了.
* If I was a man they would have given me the job. 我要是男的, 这份工作就准给我了.
* Would she tell us the truth if we asked her? 我们问她, 她会告诉我们实情么?
* If you liked (ie With your approval) I could ask my brother to look at your car. 你要同意的话, 我可以叫我弟弟来给你检查一下汽车.
* They would have been here by now if they'd caught the early train.他们要是赶上了早班火车, 现在都该到这儿了.
* I wouldn't have believed it possible, if I hadn't seen it happen. 要不是亲眼目睹, 我决不会相信能有这种事. =>Usage at unless 用法见unless.
2 when(ever) 无论何时; 当
*If metal gets hot it expands. 金属遇热则膨胀.
* She glares at me if I go near her desk. 我一走近她的办公桌, 她就瞪我.
3 (used with will and would as the first part of a sentence when making a polite request 与will和would连用, 作复合句的前部, 用以提出请求显得客气)
*If you will sit down for a few moments (ie Please sit down and) I'll tell the manager you're here. 请稍坐, 我这就通知经理说您来了.
* If you would care to leave your name, we'll get in touch as soon as possible. 请留下您的名字, 我们尽快和您联系.
4 (used after ask, know, find out, wonder, etc to introduce alternatives 用于ask、know、find out、wonder等之後, 以引导含不同可能性的情况) whether 是否
*Do you know if he's married? 他结婚没结婚, 你知道吗?
* I wonder if I should wear a hat. 我不知道该不该戴帽子.
* He couldn't tell if she was laughing or crying. 他弄不清她是笑还是哭.
* Listen to the tune see if you can remember the words. 请听这曲子--看你能不能想得起歌词来.
5 (used after vs or adjs expressing feelings 用于表示情感的动词或形容词之後)
*I am sorry if I'm disturbing you. 很抱歉, 打搅您了.
* I'd be grateful if you would keep it a secret. 如予保密, 无任感激.
* Do you mind if I switch the radio off? 我可以关掉收音机吗?
6 (also even if) (used when admitting that sth may be true or may happen 用于承认某事可能属实或可能发生) although 即使; 纵然; 虽然
*If he said that, he didn't expect you to take it personally. 即使他是那样说的, 他也并不是针对你个人而言的.
* Even if you saw him pick up the money, you can't be sure he stole it. 就算你看见是他拾起的钱, 你也不能肯定钱就是他偷的.
7 (used before an adj to introduce a contrast 用于形容词之前, 以引导一对比情况) although also 虽然; 尽管
*It was thoughtless if well-meaning. 用意虽好, 考虑欠周.
* He's a real gentleman, if a little pompous at times. 他可是个正人君子, 虽然有时略显得傲慢了些.
8 (used to express surprise, astonishment, dismay, etc 用以表示意外、惊奇、沮丧等)
*If it isn't my old friend Bob Thomson what a coincidence! 这不是我的老朋友鲍勃·汤姆森吗--真巧哇!
* If that's not the best idea I've heard in a long time! 很久没听到过这么好的意见了啊!
* If he hasn't gone and got into trouble with the police! 他竟然跟警方惹上了麻烦!
9 (used before you think, ask, remember, etc to invite sb to listen to one's opinion 用于you think、ask、remember等之前, 请对方听自己的意见)
*If you ask me, she's too scared to do it. 依我说, 她是吓得不敢去做.
* If you think about it, those children must be grown-up by now. 想想吧, 那些孩子现在一定都长大成人了.
* If you remember, Mary was always fond of animals. 记得吗, 玛丽一向喜欢动物.
10 (idm 习语) ,if and `when (used to express uncertainty about a possible event in the future 用以表示怀疑将来的事能否发生)
*If and when we ever meet again I hope he remembers to thank me. 倘若我们再有见面之时, 希望他不会忘记谢我. if ,I were `you; if ,I was/were in `your shoes/place (used to introduce a piece of advice to sb 用以引导出劝告某人的话)
*If I were you I'd start looking for another job. 我要是你, 就去另找工作了.
* If I were in your shoes, he'd soon know what I thought of him. 若是我处在你的地位, 他很快就会知道我对他的看法了. if `anything (used to express a tentative opinion or after a negative statement to say that the opposite is true 用以表达无把握的看法, 或用于否定的话语之後, 表示所说的是反话) if anything definite can be said, this is it 若能稍有把握地说出来, 其实是这样
*I'd say he was more like his father, if anything. 若非要说出像谁不可来, 我倒认为他略微像他父亲.
* He's not thin if anything he's rather on the plump side. 他可不瘦--按说还算有点胖呢. if `not
(a) (used after if and a v in the present or present perfect tense 用于if +动词（现在时态或现在完成时态）之後) otherwise 要不; 不然
*I'll go if you're going if not I'd rather stay at home. 你去我就去, 否则我宁可待在家里.
* If you've finished we can have a coffee if not, you'd better keep working. 你要是完事了, 咱们可以喝杯咖啡--不然的话, 你最好接着干下去.
(b) (used after a yes/no question to give a promise, warning, etc 用于yes/no疑问句之後, 表示许诺、告诫等)
*Are you ready? If not, I'm going without you. 准备好了吗? 要不, 我就自己去了. if `only
(a) (used to express a wish with reference to present or future time 用以表示对现时或未来的愿望)
*If only I were rich. 但愿我很富.
* If only I could swim. 要是我会游泳该多好.
* If only I knew her name. 我要是知道她的名字就好了.
* If only it would stop raining. 真希望雨能停.
* If only they would tell me what they've decided. 但愿他们能把决定告诉我.
(b) (used to express a wish that past events had been different 用以表示与过去事实相反的愿望)
*If only he'd remembered to buy some fruit. 他当时要是记得买些水果来该多好.
* If only I had gone by taxi. 假若我是乘计程车去的就好了. only if (when used at the beginning of a sentence, making the v in the following clause precede its subject 用于句首, 後接从句主谓倒装) only on condition that 只要; 只有
*Only if a teacher has given permission is a student allowed to enter this room. 只有得到教师的允许, 学生才可以进这间屋.
* Only if the red light comes on is there any danger to employees. 只要红灯一亮, 就表示有危及职工的险情.

n (infml 口)
1 uncertainty 不确定的事; 无把握的事
*If he wins and it's a big if he'll be the first Englishman to win for twenty years. 假使他赢了--是否能赢还是一大疑问--他将是二十年来第一个获胜的英国人.
2 (idm 习语) ,ifs and `buts reservations; arguments against sth 保留意见; 对某事物的辩解
*Now I'm not having any ifs and buts it's cold showers for everyone before breakfast tomorrow. 好了, 不要跟我讨价还价--明天早饭前大家都要用冷水淋浴.

NOTE ON USAGE 用法
*Both if and whether are used in reporting questions which invite yes/no answers or offer a choice between alternatives *if和whether均用于间接疑问句, 要求回答yes/no或提供选择
*(`Do you want a drink?') He asked whether/if we wanted a drink. （｀你们想喝点东西吗?＇）他问我们是否要喝点东西.
* He didn't know whether/if we should write or phone. 他不知道我们是写信好呢还是打电话好. Whether (NOT if) can be followed by an infinitive *whether後面可接不定式（if不可这样用）
*I'm not sure whether to resign or stay on. 我拿不准主意是辞职还是留任. After a preposition whether must be used 在介词之後只能用whether
*It depends on whether the letter arrives in time. 这取决于信是否来得及时. Whether is also used when the clause it begins is the subject of a sentence *whether还可用以引导主语从句
*Whether they win or lose is all the same to me. 他们是赢是输于我都一样. Whether (NOT if) can be immediately followed by `or not' *whether後面可直接用or not（if不可这样用）
*I'll be happy whether or not I get the job (compare 试比较
*I'll be happy whether/if I get the job or not 我能不能得到那份工作都一样高兴).

If
a famous poem by Rudyard Kipling which starts with the words "If you can keep your head when all about you/Are losing theirs and blaming it on you...". It describes the qualities of character that some people think of as typically English, such as the ability to remain calm in difficult situations.