if

Từ điển American Heritage Dictionary 4th

if (ĭf)conj.
1.
a. In the event that:
If I were to go, I would be late.
b. Granting that:
If that is true, what should we do?
c. On the condition that:
She will play the piano only if she is paid.
2. Although possibly; even though:
It is a handsome if useless trinket.
3. Whether:
Ask if he plans to come to the meeting.
4. Used to introduce an exclamatory clause, indicating a wish:
If they had only come earlier!n.
A possibility, condition, or stipulation:
There will be no ifs, ands, or buts in this matter. [Middle English, from Old English gif. See i- in Indo-European Roots.] 
Usage Note: 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: We shall soon learn whether (or if) it is true. 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: If America were still a British colony, we would have an anthem that human voices could sing. If I were the President, 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: 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. 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: If there is anything I can do to help, I should be happy to do so. He would always call her from the office if he was (not were) going to be late for dinner.·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: We wondered if dinner was (not were) included in the room price. Some of the people we met even asked us if California was (not were) an island.·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. See Usage Note at should. See Usage Note at wish.