Whatthedoctorsreallydoubtismymotherwillrecoverfromtheseriousdiseasesoon.为什么用weither不用if

Question

What the doctors really doubt is ____my mother will recover from the serious disease soon.
为什么用weither不用if

Answer 1

whether可以引导从句,作主语、表语或同位语,而if不能.如：
正：It was uncertain whether he would come.
误：It was uncertain if he would come.
正：His first question was whether Tom had arrived yet.
误：His first question was if Tom had arrived yet.
正：We must consider the question whether we will take these measures.
误：We must consider the question if we will take these measures.