英语中if的用法

1. (表示条件)如果,主句用将来时，if从句用现在时表示将来
He will come if you invite him.
如果你请他,他会来的。

2. (表示虚拟)假如,要是
1）时态：可以表示过去，现在和将来的情况。

它的基本特点是时态退后。

a. 同现在事实相反的假设。

句型：主句
should( would) +
If they were here, they would help you.
b. 表示于过去事实相反的假设。

句型：主句
should(would) have+ 过去分词
If she had worked harder, she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday, he would have prevented me from going.
If he had come , I should / would have told him about it.
含义：He did not come , so I did not tell him about it.
If he had not been ill and missed many classes, he would have made greater progress. 含义：He was ill and missed many lessons, so he did not make greater progress.
c. 表示对将来的假想
句型：主句
should+
were+ would +
should+ 动词原形
If you succeeded, everything would be all right.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
3. 是否= whether 连接
I wonder if she is ill.
不知她是否病了。

conj.
1.(表示条件)如果
He will come if you invite him.
如果你请他,他会来的。

2.(表示虚拟)假如,要是
If I were you, I would never do that.
如果我是你,我决不做那件事。

3.(表示让步)即使
We'll go even if it rains.
即使下雨我们也要去。

4.是否
I wonder if she is ill.
不知她是否病了。

5.(表示)每一次...的时候(总是会)
If I feel any doubt, I inquire.
我一感到有疑问就随时询问。

if作为连词的时候，一般用于或者是或
if引导的：If he comes, please tell me.（如果，假如）
主句和从句位置可以互换，时态：
变成：Please tell me,if he comes.
中：I want to know if（是否）he comes.
可改成：I want to know whether he comes.
当句子中有or not是只能用whether——I want to know whether he comes or not. 时态：1.主过从过 2.主句，从句任何时 3.用
中，if 解释为即使
名词cn. 条件;设想
There are too many ifs in his offer.
他的提议中条件太多了。