什么时候用wether什么时候用if

一、引导宾语从句
这时两者的含义区别很小,一般可通用.
I don't know whether/if they will come to help us.
但是,当whether与or not连成一个词组时,whether不可换用if.
I don't know whether or not they will come for our help.
若whether和or not不连在一起,在口语中可以用if取代whether,当然也可以用whether.
I don't care if/whether your car breaks down or not.我不在乎您的车是否是会出故障.
此外,还有三种情况值得注意：
(1)在介词后面只能用whether,不能用if.
I am not interested in whether you'll come or not.
(2)在动词不定式之前,只能用whether,不能用if.
He doesn't know whether to stay or not.
(3)在及物运动discuss后的宾语从句中,只能用whether,不能用if.
We discussed whether we should make a change in our plan.
二、引导主语从句
只能用whether,不能用if.
It is unknown whether he will come.
Whether the news is true remains a question.
三、引导表语从句
只能用whether,不能用if
The question is whether they can take our advice.
四、引导同位语从句
常用whether
The question whether he'll come is unknown.
五、可以用来引导一个否定的宾语从句,whether则不能用来引导否定的宾语从句,因为它表示正反两方面的选择意义比较强.
Tell me if it is not going to rain,please.
六、if除引导宾语从句外,还可以引导条件状语从句,作“是否”解.在容易引起混淆产生歧义的情况下,就不可用if,而须用whether.
Please let me know if you intend to come.这个句子有两种解释：A：“请让我们知道你是否想来.” B：“如果你打算来,请让我们知道.”在书面语中,如果我们想表达的第一种含义,就得用whether来改写第一句.
Please let me know whether you intend to come.
总之，whether在大多数情况下都可以用,而if则不然,有wether和if时一般选wether不会有错,但是如果所需的意思是"如果",就只能选if。

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 ：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.

3.是否 = whether 连接宾语从句
I wonder if she is ill.
不知她是否病了.