第一在动词不定式之前只能用whether，例如I can't decide whether to stay.我不能决定是否留下来。

　　第二个在whether or not的固定搭配中只能用whether，例如I want to know whether it's good news or not.

　　第三在介词后，也只能用whether，His father is worried about whether he lose his work.他的父亲担心是否会失去工作。

　　第四宾语从句放在句首表示强调时，只能用whether，wherther 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。