（1）在否定句和疑問句中,doubt后面接that引導(dǎo)的賓語從句.例如：
I don't doubt that he can finish the task on time. 我相信他能按時(shí)完成任務(wù).
Do you doubt that she will succeed? 你懷疑她會成功嗎?
（2）在肯定句中,doubt后面一般接whether或if引導(dǎo)的賓語從句.例如：
I doubt whether they can swim across the river. 我懷疑他們能否游過河去.
He doubts if she will keep her word. 他不敢肯定她是否會遵守諾言.
注意：在肯定句中,doubt后面也可以接that引導(dǎo)的賓語從句,但表示疑慮較大或“不相信”.例如：
We doubt that they can complete the work ahead of time. 我們不相信他們能提前完成這項(xiàng)工作.
I doubt that he will come. 我看他未必會來.