7、定概念宾语从句时,只能用if,而不用whether。如:He asked me if I hadn't finished my work. 2)当doubt是肯定形式时,用whether,如:Idoubtwhetherit’strue.Ioftendoubtwhether/ifitisworthworkingsohard.但是,当doubt是否定形式时,则不用whether或if,要改用that,因为句子本身已没有“怀疑”或“选择”之意。比较:Idon’tdoubtthatyouarehonest.我不怀疑你的诚实。Shen