---Really, I don’t know _____ to join the army or to find a job after my graduation.
---Why don’t you _____ your parents’ advice ? They will surely help you.
A. whether; follow B. if ;give C. whether; seek D. if ;take

if和whether都可作"是否"解，但下列情况下只能用whether而不能用if：
☆后面紧随不定式或or not时（有人认为只要后面有or not就不能用if）,如：1）She wondered whether to wait for them or go on.2)I ask you whether it is true or not.
☆用于介词后时，如I have settled the question of whether I'll go back.
☆引导主语从句、表语从句、同位语从句时，如：1)It doesn't matter whether he'll come or not.2）The problem is whether we need it.3)I have no idea whether they are there.
☆宾语从句前置时，如：Whether he has a house,I want to know.
☆直接引语中的问句里含有if引导的条件句时，如：Bill asked whether,if I got the job,I'd move to New York.
☆与某些动词如discuss连用时：We discussed whether we should close the shop.
但引导否定结构时不能用whether而只能用if：Tell me if it doesn't rain.


参考文献：1.胡鑫镛：《英语语法五十题》，2.雷馨：《英语分类句型》》