1.引导宾语从句,意为"是否"相当于whether
I don't know if/whether she will come.
我不知她是否会来。
2.引导条件状语从句,意为"如果" ,表示条件,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。
3. 表示一种虚拟假设,引导虚拟条件从句,意为”假如,要是”
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 my lawyer had been here last Saturday, he would have prevented me from going.
If he had come yesterday, I should / would have told him about it.
含义：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.
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： 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.
温馨提示：答案为网友推荐，仅供参考