在doubt疑问句“有疑问吗”和否定句“没疑问”或肯定句中作不信解时只能使用that
Do you doubt that he will win the game?你对他赢得比赛有疑问么?
I don't doubt that he will win the game.我毫不怀疑他会赢得比赛.
I doubt that he will win the game.我不相信他会赢得比赛.
doubt怀疑的肯定句中用if
I doubt if he will win the game.我怀疑他是否会赢得比赛.