★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.