if作为从属连词，可引导条件状语从句、让步状语从句和名词从句。另外，if还能用作名词，表示不确定的情况。

用法一、（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。

用法二、（表示虚拟）假如，要是

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：条件从句 主句

一般过去时 should(would)+动词原形

If they were here,they would help you.

b、表示于过去事实相反的假设

句型：条件从句 主句

过去完成时 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 he had come yesterday,I should / would have told him about it.=He did not come yesterday,so I did not tell him about it.

If he had not been ill and missed many classes,he would have made greater progress.=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：条件从句、主句

一般过去时 should+动词原形

were+不定式 would+动词原形

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.

用法三、表示是否，等同于whether，连接宾语从句

I wonder if she is ill.

不知她是否病了。