whether可以放在介词后面作宾语，而if 不可以，只能用whether主要有以下几种情形：
1、强调两方面的选择,特别是句中有or not时。
例：Let me know whether you can come or not.
2、宾语从句前置时。
例：Whether this is true, I can\'t say.
3、引导主语、表语、同位语从句时。
例：Whether the football game will be played depends on the weather. （主语从句）
The question is whether he can do it. （表语从句）
The question whether we should go with them must be decided at once. （同位语从句）
4、在介词之后作介词的宾语时。
例：I haven\'t settled the question of whether I\'ll go back home.
5、在不定式前与不定式一起组成词组时。
例：Whether to go or stay is still a question.
6、在discuss, decide 等动词后作宾语时。
例：We discussed whether we should go there by plane.