if当是否引导什么从句

　　Please let me know how many are coming, if any.

　　请告诉我，如果有人要来的话，有多少？

　　That was a disaster if ever there was one!

　　那确实是场灾难！

　　Put your hand up if you know the answer.

　　知道答案就举手。

　　If you love each other, why not get married?

　　要是你们彼此相爱，干吗不结婚呢？

　　If you don't believe me, go and see for yourself!

　　要是不信我说的，你自己去看看！