doubt既可用作动词,也可用作名词.现将其用法归纳如下.
一、doubt用作动词
1.doubt+名词或代词.例如：
I doubt his word.我怀疑他的话.
They doubted him before.他们以前曾经怀疑过他.
2.doubt+宾语从句
（1）在否定句和疑问句中,doubt后面接that引导的宾语从句.例如：
I don”t doubt that he can finishthe task on time.我相信他能按时完成任务.
Do you doubt that she willsucceed?你怀疑她会成功吗?
（2）在肯定句中,doubt后面一般接whether或if引导的宾语从句.例如：
I doubt whether they can swimacross the river.我怀疑他们能否游过河去.
He doubts if she will keep herword.他不敢肯定她是否会遵守诺言.
注意：在肯定句中,doubt后面也可以接that引导的宾语从句,但表示疑虑较大或“不相信”.例如：
We doubt that they can completethe work ahead of time.我们不相信他们能提前完成这项工作.
I doubt that he will come.我看他未必会来.
二、doubt用作名词
1.doubt常与about/of/as to/on等介词连用.例如：
There is no doubt aboutit.此事无可怀疑.
I have no doubt of hisability.对于他的能力我毫不怀疑.
She has her doubts as to thisbeing true.她怀疑这事是否真实.
No one has any doubt on thispoint.这一点没有人会怀疑.
2.doubt后面接同位语从句
（1）doubt用在否定句中,后面接that引导的同位语从句.例如：
I have no doubt that he willsucceed.我相信他会成功的.
There is no doubt that they willagree with you on this matter.毫无疑问,他们在这件事上会同意你的意见的.
（2）doubt用在肯定句中,后面接whether引导的同位语从句.注意不可以用if替换whether.例如：
He had his doubts whether theywould give him another chance to have a try.他不能肯定他们是否会再给他一次机会试一试.
There is some doubt whether theirfootball team will win the match.他们的足球队会不会赢得这场比赛还不能肯定.
3.由doubt组成的一些短语
（1）indoubt怀疑；拿不定主意.例如：
We are in doubt(about) what to donext.我们不知道下一步要做什么.
When in doubt about the meaningof a word,consult the dictionary.当你对一个词的意义没有把握时,就查一下词典.
（2）no/without/beyond doubt无疑地；必定；当然.例如：
It is no doubt difficult.这无疑是困难的.
No doubt he didn”t mean to hurtyou.他肯定不是有意伤害你的.
Without doubt this is thebest.无可置疑,这是最好的.
Beyond doubt he will give yousome advice on your study.毫无疑问,他会给你的学习作一些指导的.
（3）makeno doubt of 对……毫不怀疑.例如：
We make no doubt of hissincerity.我们对他的诚意毫不怀疑.
She made no doubt of what he hadsaid.她对他所说的话毫不怀疑.
（4）throw/cast doubt on使人对……产生怀疑.例如：
Galileo”s experiments threwdoubt on Aristotle”s theory of falling objects.伽利略的实验使人们对亚里士多德的落体理论产生了怀疑.
What he had done cast doubt onhis honesty.他的所作所为使人们对他的诚实产生了怀疑.
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