1.whether和if均可用来引导主语从句，但if引导的从句不能位于句首。
如：Whether they win or lose is all the same to me.他们是赢是输於我都一样.（此时whether不能换作if ）It's doubtful whether there'll be any seats left.说不上还有没有空座位了.(此句whether能换作if)2.whether和if均可用来引导宾语从句。
如：①I don't know whether/if I will be able to come.我不知道我是否能来.②She asked me whether/if I was interested in working for her.她问我是否有兴趣为她工作。
③We'll be told tomorrow whether we should take the exam or not.我们明天才知道是不是应该参加考试.④I asked him whether he had done all the work himself or whether he had had any assistance.我问他这些工作都是他自己做的还是别人帮他做的.( 注意: 在两个供选择的从句之间若用or字, 则须重复whether一字. )注:⑴当强调宾语从句并将其提前时只能用whether。
如:Whether I'll have time to go with you I am not sure.我很难说我们是否有时间跟你们一起去。
⑵从句作介词宾语时只用whether不用if。
如:She was undecided about whether she should accept his offer.她拿不定主意应该不应该接受他的好意⑶当whether和or not连用时，只用whether不用if。
当or not放在whether引导的从句句尾，则可以用if来替换。
如: I don’t know whether or not he will come.我不知道他是不是回来。
I'll be happy whether/if I get the job or not我能不能得到那份工作都一样高兴⑷当whether和or 连用表示明确的选择时，只用whether不用if。
如:He asked me whether I wanted to go there by train or by bus.他问我是想乘火车还是坐公共汽车去那里。
He hesitated about whether he should drive or take the train.他开车去还是坐火车去, 犹豫不决注:whether 和or 连用还可以引导让步状语从句,此时也不用if。
如：We'll go on with the work, whether we can find the necessary tools or not.不管我们能否找到所需的工具,我们要把这件工作做下去。
Whether or not it rains / Whether it rains or not, we're playing football on Saturday. 无论下不下雨, 我们星期六一定踢足球.⑸动词discuss，decide,consider等后习惯上也常用Whether引导从句，不用if。
They discussed whether they should leave at once.他们讨论了是否应该马上离开。
We discussed whether the house should be sold. 我们讨论了这房子是否应卖掉。
3.表语从句用whether引导不用if.如:①The doubt was whether you had broken the beautiful vase.疑问是你是否打坏了这只漂亮的花瓶。
4.同位语从句用whether引导不用if.如:①The question whether we'll build another lecture building hasn't been settled. 是否要另建一幢教学楼的问题还未决定。
5. 在动词不定式之前，只能用whether，不能用if。
如:①The question is whether to go to Munich or Vienna. 问题是去慕尼黑还是去维也纳.②She doesn't knows whether to get married now or wait. 是否现在结婚或是等待她不知道。
③We were wondering whether to go today or tomorrow. 我们弄不清是今天走还是明天走.④I'm not sure whether to resign or stay on. 我拿不准主意是辞职还是留任. Answers: BADDC ABCAB ABADD B强化练习：1. The question has not been answered ____ Taiwan will be included on the Olympic torch relay route. A．that B．whether C．which D．if2. You have not yet answered my question _____ I can join in the party tonight or not.A. whetherB. ifC. thatD. which3．—I d like to invite you to a party this evening.—Thank you, but _____ I’ll be free I m not sure at the moment.A. whileB. ifC. whenD. whether4．---What are you anxious about? ---___________A. If we succeedB.That we can succeedC. Do we succeedD.Whether we can succeed5. Only when _____ the painting _____ decide ______the painting is worth buying.A. he sees, he can ,whetherB. does he see, can he, ifC. he sees, can he ,whetherD. sees he, he can,that6. _____ English is his favourite subjectA. I asked him ifB. I asked him that whetherC.I asked him thatD. I asked him which7. _____ they’ll go to the Summer Palace ____ the weather. A. If; depends onB. Whether; depends uponC. Whether; depended onD. If; depended upon8.It isn’t quite _____ ______ he will be present at the m eeting.A. sure; whetherB. sure; ifC. certain; whetherD. certain; that9. _____ the 2008 Olympic will be held in Beijing is not known yet.A. WhetherB. IfC. WheneverD. That10. No one was sure _____ he would be asked to speak next year.A. whenB. whetherC. whatD. how11. It is still a question to us all _____ there is any living thing in some other places in the space. A. whether B. which C. what D. where12. I doubt _____ he will come here on time.A. thatB. whetherC. whenD. where13. The point is not who said the words, _____ they are true or not.A. but whetherB. and whetherC. but howD. and how14. “What did he ask you?”“_____ I would be late.”A. ThatB. WhenC. WhereD. Whether15. He treated all the people around him,______he knew or he didn't know,______.A. if, friendlyB. whether, friendlyC. if, in a friendly wayD. whether, in a friendly way16. At the meeting, we discussed _____ we should employ more workers.A. ifB. whetherC. thatD.不填Answers: BADDC ABCAB ABADD B因为只有两个句子，两个句子需要一个连词就可以了，如果是三个句子的话才需要两个连词。
例如：we have decided that if it is fine tomorrow,we will go out to play. whether or not是固定搭配。
if和whether有时可以互用，但有or not,一定是whether。