1. [后接宾语从句时，如主句为肯定句时用 if 或 whether 连接，如主句为否定句或疑问句时用 that 连接] 不信任，不相信，怀疑，疑惑：I never doubted you; I was sure you would bring me the money.我从不怀疑你的，我肯定你会给我带来这笔钱的。I doubt whether (或 if) he will want to participate.我怀疑他是否愿意参加。
2. [后接宾语从句时，如主句为肯定句时用if 或 whether 连接，如主句为否定句和疑问句时则用that 连接]不能肯定，拿不准，对…没准儿，对…吃不准：I doubt whether (或 if) he'll change his mind.我拿不准他是否会改变主意。I wouldn't doubt that he'd want to help.我肯定他愿意帮忙。
3. [后接宾语从句时，用that连接]认为…未必可能：I doubt that he was invited.我看他未必受到了邀请。I don't doubt that she'll come我肯定她会来的。
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1. [后接宾语从句时，如主句为肯定句时用 if 或 whether 连接，如主句为否定句或疑问句时用 that 连接] 不信任，不相信，怀疑，疑惑：I never doubted you; I was sure you would bring me the money.我从不怀疑你的，我肯定你会给我带来这笔钱的。I doubt whether (或 if) he will want to participate.我怀疑他是否愿意参加。
2. [后接宾语从句时，如主句为肯定句时用if 或 whether 连接，如主句为否定句和疑问句时则用that 连接]不能肯定，拿不准，对…没准儿，对…吃不准：I doubt whether (或 if) he'll change his mind.我拿不准他是否会改变主意。I wouldn't doubt that he'd want to help.我肯定他愿意帮忙。
3. [后接宾语从句时，用that连接]认为…未必可能：I doubt that he was invited.我看他未必受到了邀请。I don't doubt that she'll come我肯定她会来的。