doubtful

questionable, doubtful, ambiguous, uncertain

形容词 doubtful:

1. open to doubt or suspicion- Karen Horney
   同义词：dubious, dubitable, in question
2. fraught with uncertainty or doubt
   同义词：dubious
3. unsettled in mind or opinion
   同义词：tentative

1. The ethics of his decision are doubtful.
   他的这一决定是否合乎道德规范值得怀疑。
2. It is doubtful whether he can read my fortune or not.
   他能否算出我的运气还是个疑问。
3. I am doubtful about the propriety of granting such a request.
   我怀疑答应这项要求是否合适。
4. A doubtful claim; doubtful prospects.
   令人怀疑的索赔案; 令人可疑的前景
5. Because of a long drought, the farmers are doubtful about the prospect of a good yield.
   因为长时间的干旱，农民们对于好收成的前景是怀疑的。

doubt.ful

adj.（形容词）
Subject to or causing doubt:
可疑的，令人怀疑的：
例句：
a doubtful claim; doubtful prospects.
令人怀疑的索赔案；令人可疑的前景

Experiencing or showing doubt:
疑惑的，怀疑的：
例句：
Doubtful that the cord would hold, we strengthened it.
绳索恐怕不能支撑，让我们使它坚固一点

Of uncertain outcome; undecided.
不能确定的：未决的；未定的
Raising doubts as to legitimacy, honesty, or respectability; suspicious:
令人质疑的：合法性、诚实性或名声有问题的；不可靠的：
例句：
the candidate`s doubtful past.See Usage Note at doubt
这位候选人过去名声令人质疑参见 doubt

【引伸】
doubt.fully
adv.（副词）
doubt.fulness
n.（名词）
<参考词汇><同义词>doubtful,dubious,questionableThese adjectives express a degree of uncertainty or reservation about someone or something.
这些形容词都表示不确定的程度或对某人、某事的异议。
Doubtful suggests pronounced lack of certainty:
Doubtful 表示明显的不确定性：
例句：
It is doubtful whether she will be admitted to graduate school.
她能否考上研究生很可疑。

例句：
He is an author of doubtful reputation.
他是一位名声不太好的作家。

Dubious expresses uncertainty less directly and less forcefully;often it suggests hesitancy, suspicion, or mistrust:
Dubious 表示的不确定性不那么直接和不那么强，通常它含有犹豫不决、猜疑、不信任的意思：
例句：
dubious about agreeing to go;
迟疑不决到底同意还是不同意去；

例句：
a painting of dubious value.
价值没法断定的画。

Questionable can mean merely open to question (
Questionable 意思是.值得疑问的.：（
例句：
Whether this is true is questionable); often, however, it implies dubious morality, respectability, or propriety:
这是不是真的倒成问题）； 然而，它通常暗指道德、名声或行为等的有问题：

例句：
questionable dealings;
不正派的行为；

例句：
a questionable stockbroker.
不正当的股票经纪人