if的用法（英语语法）

http://www.english-at-home.com/grammar/if-sentences/

There are four main types ofifsentences in English, often called conditional sentences.

These sentences are in two halves, with theifpart in one half and the other part where you can use words such ascan,will,may,might,couldandwould.

If + present form + present form

"If youheatice, itmelts."

In this type of sentence, you could usewheninstead ofif. It's always true that when you heat ice it melts. This is why this type of sentence is sometimes called a zero conditional.

If + present form, + will, can or may

"If Iamlate, Iwillcall you."

"If youneedme, youcancall me at home."

"If itgetsany hotter, wemayhave a thunder storm."

In these sentences (or first conditional sentences), there is a strong possibility that the first part (coming afterif) is going to happen. The second part says what will happen as a result.

If + past form + would, could or might

"If Igota pay rise, Iwouldbuy a new car."

"If youleftyour job, youcouldtravel around the world."

"If youwerenicer to him, hemightlend you the money."

In these sentences, the first part withifshows that the event is unlikely to happen. In English, we often use this type of sentence (called a second conditional) to talk about hypotheses, or imaginary future events.

For example, "If I was President of the United States, I would change some laws." But I know that I'll never be the President of the USA – I'm just saying what I would do if I was in his/her position. Note: in American English, it is correct to use "if Iwere…" In British English, it's more common to say "if Iwas…"

If + past perfect + would/might/could have done

"If Ihad revised, Iwould have passedmy exams."

"If wehad goneout earlier, wemight have gotto the cinema on time."

"If youhad toldme there was a problem, Icould have helped."

In these sentences (or third conditional sentences), the first part of the sentence withifdidn't happen. So there is no possibility of the second part of the sentence happening. I didn't revise, so I didn't pass my exams and there is nothing I can do about it now. English speakers use this type of sentence to show how things could have been different.