We haven't decide ___ to go to Beijing(whether,if)

问题描述:

We haven't decide ___ to go to Beijing(whether,if)

whether
我们没决定是否去北京if也表示是否,为什么不用if?在动词不定式之前,只能用whether,不能用if。例如:①He doesn't know whether to stay or not. 他不能知道是否要留下来。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