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.