doubtful: is likely to impute worthlessness, unsoundness, failure, or uncertainty: still ~ about the cause of the explosion. （impute归咎指责认为不好suggests ascribing something that brings discredit by way of accusation or blame，如tried to impute evil motives to my actions认为我动机邪恶，来自词根PUT表示想to think, consider, believe，又如repute名誉compute计算dispute争端putative公认的推定的）
dubious: stresses suspicion, mistrust, OR hesitation: ~ about the practicality of the scheme. （suspicion怀疑强调"没有信心lack of faith in truth, fairness..."，mistrust表示由于suspicion而不信任，含义很接近，详见2017-04-11 怀疑uncertainty-doubt） （hesitate犹豫迟疑implies pausing before deciding or acting or choosing 指在决定行动选择前停顿，如hesitated before answering the question，结合上下文这里表示疑虑） (dubious有2种用法 1、人感到怀疑to feel uncertain or suspicious，如I was rather dubious about the whole idea我对这整个想法持怀疑态度，如本例，怀疑方案是否可行 2、事物可疑not honest, reliable, safe，如they indulged in some highly dubious business practices他们采取了一些极为可疑的商业手段，如this claim seems to us to be rather dubious在我们看来这种说法非常可疑)
problematic: applies especially to things or situations whose existence, meaning, or realization is highly uncertain: whether the project will ever be finished is ~. problematic暗指的问题（existence,meaning,realization）比下一词questionable暗指的问题(propriety, well-grounded suspicion)"困难复杂" （problem问题来自词根PROB证明=to prove，如probe探测器probation见习考察期probity诚实正直approbation认可批准reprobate堕落的人；question问题来自词根QUIS寻求=to seek, obtain，又如require要求inquire问讯acquisitive贪婪的。两个词都表示"问题"，problem是比较"困难复杂"的，而question是一般的，变成problematic和questionable后仍有这一区别）
questionable: may imply no more than the existence of doubt but usually suggests doubt about propriety or well-grounded suspicions: a real estate agent of ~ honesty. (如her motives for helping are questionable她帮忙的动机令人生疑，如the conclusions that they come to are highly questionable他们得出的结论大有问题)
doubtful不确定: 指认为无价值不合理难成功没把握，可见较通用，dubious怀疑疑虑: 强调怀疑不信疑虑（hesitation这里表示疑虑），problematic有问题不确定: 对事物"是否存在有无意义能否实现"等不确定，问题困难复杂，questionable有问题可疑: 可表示一般的怀疑，但常指有一定根据、道德操守方面的怀疑
记忆方法: 1）首字母DDPQ想成抵挡欺骗<==怀疑 "疑"本义是迷惑，"疑, 惑也"，引申为疑问，猜疑，犹豫，该字甲骨文像"人持杖出行时仰望天色"的样子。如《尚书》中: "任贤勿贰,去邪勿疑"任用贤才不要三心二意,铲除邪恶不要犹豫，这里表示犹豫迟疑
2）怀疑的意思是没有信心mean not affording assurance of worth, soundness, or certainty of something or someone. （sound有理有据implies a basis of flawless reasoning or solid gounds无瑕的推理,坚实的基础，如a sound proposal for combatting terrorism打击恐怖主义的合理建议）
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