IF 或 i.f.
1. Intermediate frequency.
1. In the event that:
If I were to go, I would be late.
2. Granting that:
If that is true, what should we do?
3. On the condition that:
She will play the piano only if she is paid.
4. Although possibly; even though:
It is a handsome if useless trinket.
Ask if he plans to come to the meeting.
6. Used to introduce an exclamatory clause, indicating a wish:
If they had only come earlier!
7. A possibility, condition, or stipulation:
There will be no ifs, ands, or buts in this matter.
8. Middle English
9. from Old English gif * see i-
源自 古英语 gif *参见 i-
10. In informal writing both if and whether are standard in their use to introduce a clause indicating uncertainty after a verb such as ask, doubt, know, learn, or see:
在非正式的写作中，可以使用if 或 wether 两个词来表达不定性，两者都算标准，例如 ask,doubt,know,learn, 或 see这样的动词后：
We shall soon learn whether (or if ) it is true.
我们很快就会知道这是否（或 if ) 是真实的。
11. In such contexts, however, the use of if can sometimes create ambiguities. Depending on the intended meaning, the sentence Let her know if she is invited might be better paraphrased as Let her know whether she is invited or If she is invited, let her know. · In conditional sentences the clause introduced by if may contain either a past subjunctive verb (if I were going ) or an indicative verb ( if I am going; if I was going ), depending on the intended meaning. According to the traditional rule, the subjunctive should be used to describe an occurrence that is presupposed to be contrary to fact, as in if I were ten years younger or if Napoleon had won at Waterloo. The main verb of such a sentence must then contain the modal verb would or (less frequently) should:
不过在这种情况下，用if 就可能会造成歧义。 Let her know if she is invited 这句话最好根据所要表达的意思， 分开说成告诉她她是否被邀请了 或 如果她被邀请了，就让她知道 两句话。 用if 引起的条件状语从句中， 可以含有一个过去式的虚拟动词(倘若我去 )或一个一般陈述动词( 如果我就去；如果我去了 )， 这得依据所要表达的意思而定。根据正统的原则，虚拟语气用来表达一个与事实相反的假设，例如如果我再年轻十岁 或者 如果拿破仑在滑铁卢战役中获胜。 于是在这样的句子里，主要动词必须包含情态动词会 或（在较少情况下） 应该：
If America were still a British colony,
12. we would have an anthem that human voices could sing.
If I were the President,
13. I should (or would ) declare November 1 a national holiday. When the situation described by the if clause is not presupposed to be false, however, that clause must contain an indicative verb, and the choice of verb in the main clause will depend on the intended meaning:
我就（或 会 ) 宣布十一月一日为国家休假日。 而只要if 引起的从句描述的条件不是事先就被认为是不可能的， 从句中就必须使用直陈动词，而主句中词的形式就要依所要表达的意义而定：
If Hamlet was really written by Marlowe, as many have argued, then we have underestimated Marlowe's genius.
If Kevin was out all day, then it makes sense that he couldn't answer the phone.
14. Note also that the presence of the modal verb would in the main clause should not be taken as a sign that the verb in the if clause must be in the subjunctive, if the content of that clause is not presupposed to be false:
另须注意，在主句中使用情态动词would 并不表明 if 从句中一定使用虚拟语气，因为从句的内容并不一定原先就认为是错误的：
If I was (not were ) to accept their offer—which I'm still considering—I would have to start the new job on May 2.
如果我去（不是 were ) 接受他们的盛情——我还在这样考虑——那么我必须在5月2日开始新工作。
He would always call her from the office if he was (not were ) going to be late for dinner. ·
如果他（不是 were ) 晚餐会迟到，就会从办公室打电话给她。 ·
15. Again according to the traditional rule, the subjunctive is not correctly used following verbs such as ask or wonder in if clauses that express indirect questions, even if the content of the question is presumed to be contrary to fact:
另外还是根据传统规则，在动词ask 或 wonder 中 if 引导的间接问句， 即使其内容被认为与事实相反，也不能使用虚拟语气：
We wondered if dinner was (not were ) included in the room price.
我们想知道晚餐费用是否被（不是 were ) 包括在房间费中。
Some of the people we met even asked us if California was (not were ) an island. ·
我们碰到的一些人甚至问我们加利福尼亚是否是（不用 were ) 一个岛屿。
16. With all deference to the traditional rules governing the use of the subjunctive, it should be noted that a survey of the prose of reputable writers over the past 200 years would reveal a persistent tendency to use the indicative was where the traditional rule would require the subjunctive were. A sentence beginning If I was the only boy in the world, while not strictly correct, is wholly unremarkable. But the corresponding practice of using the subjunctive in place of the indicative may be labeled a hypercorrection. · In spoken English there is a growing tendency to use would have in place of the subjunctive in contrary-to-fact clauses, as in if I would have been the President, but this usage is still widely considered incorrect.See Usage Note at doubt, should, wish
在与指导虚拟语气使用方法的传统规则相冲突的一系列不同用法中，我们应注意到一项对过去200年散文作家的调查显示，在传统语法要求使用虚拟式的were 的情况下，有一种持久的使用直陈式 was 的趋势。 一句用假如我是世界上唯一的男孩 开头的句子不会引起人们的注意。 而一定要把其中的直陈式换成虚拟式就显得矫正过了头。在英语口语中，在与事实相反的从句中，使用would have 一词的趋势有所增加， 例如如果我会是总统， 但这种用法仍然被大多数人认为是不正确的参见 doubt， should， wish