★if不可引导位于句首的主语从句，只可用whether；如果用it作形式主语，而将主语从句置于句末时，则可以用if。如：

Whether Marcus will succeed remains to be seen.

It remains to be seen whether / if Marcus will succeed.

二、表语从句

★用作表语的从句叫做表语从句。如：

The trouble is that they are short of money.

The problem is who we can get to replace Monica.

The question is when Sandra will start the project.

★后跟表语从句的除be外，还有appear, become, look, remain, seem, sound等连系动词。如：

The boy remained where you told him to stand for over half an hour.

★在advice, suggestion, order等表示建议、劝告、命令等含义的名词后的表语从句中，谓语要用"should +动词原形"，should可以省略。如：

George's suggestion is that we (should) stay calm.

★whether可以引导表语从句， if一般不可以。如：

His first question was whether Mike had arrived yet.

★as if也可引导表语从句。如：

All this was over 20 years ago, but it's as if it was only yesterday.