whether及其引导的成分可放于介词之后,作介词的宾语,但if不能?

if和whether都可译为\"是否\",二者引导宾语从句时通常可以互相替换,口语中多用if代替whether.例如：Ask if/whether he plans to come to the meeting.而以下几种情况中,只能用whether:
1.强调两方面的选择,特别是句中有or not时.如：
Let me know whether you can come or not.
2.宾语从句前置时.如：
Whether this is true,I can\'t say.
3.引导主语、表语、同位语从句时.如：
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.如：
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.如：
Whether to go or stay is still a question.
6.在discuss,decide 等动词后作宾语时.如：
We discussed whether we should go there by plane.
值得注意的是：if还可以引导条件状语从句,是\"假如；如果\"之意,而whether可以引导让步状语从句,是\"不管；无论\" 之意.如：
If you ask him,he will help you.
I shall go,whether you come with me or stay at home.
一、if或whether引导宾语从句时,作“是否”讲,常放在动词ask 、see 、say 、know和find out等后面.一般情况下,两者常可换用,在口语中多用if.如：
例1 Nobody knows whether (if) it will rain tomorrow.没有人知道明天是否下雨.
例2 Lucy asked whether (if) they had a cotton sweater .露西问他们是否有棉制的羊毛衣.
二、if或whether引导宾语从句时,要注意三个方面,即连词、语序和时态.
1 if或whether不能和that 或其它连词（副词）同时使用,也不能省去.如：
例3 我不知道他今天是否会来.
[误] I don’t know that if(whether) he will come here today .
[正] I don’t know if(whether) he will come here today .
例4他走过来看看猫是否出了毛病.
误] He came to see what if (whether) there was wrong with his cat .
[正] He came to see if (whether) there was wrong with his cat .
2 if或whether引导的宾语从句时,虽具有疑问意义,但从句语序应用陈述句语序.如：
例5 Did you know the way to the hospital The old woman asked me .
The old woman asked me .if (whether) I knew the way to the hospital .老妇人问我是否知道去医院的路.
3 if或whether引导的宾语从句应和主句的时态保持一致.即主句为一般现在时,从句用任一种时态；主句为一般过去时,从句则用过去时中的任一种时态.如：
例6 I don’t know if (whether) he has come here .我不知道他是否来过这儿.
例7 Tom asked if (whether) I had read the book .汤姆问我是否看过这本书 .
三、if和whether的区别：
1 在动词不定式之前只能用whether .如：
例8 I can’t decide whether to stay.我不能决定是否留下.
2 在whether …… or not 的固定搭配中.如：
例9 I want to know whether it’s good news or not .我想知道是否是好消息.
3 在介词后,只能用whether.如：
例10 His father is worried about whether he lose his work .他的父亲担心是否会失去工作.
4宾语从句放在句首表示强调时,只能用whether .如：
例11 Whether they can finish the work on time is still a problem .他们是否能准时完成这项工作还是个问题.
5用if会引起歧义时,只用whether.如：
例12 Could you tell me if you know the answer
这句话有两种意思：“你能告诉我是否知道答案吗?”或“如果你知道答案,请告诉我,好吗?”.如用whether可避免歧义.