dubious（adj） ～ about sth/doing sth （尤作表语）半信半疑，可疑；（贬）名声不大好的，不可靠的； 结果未定的，不能确定的；
I remain dubious about his motives。
我对他的动机心存疑虑。
The testimony given by him is dubious.
他所作的证据是可疑的。
You burst in on her doing something dubious and she is trying to neutralise it by involving you.
你在她做可疑事情的时候突然出现，她是在试图把你卷进来，从而息事宁人。
But this is a highly dubious assumption. Government bodies still have to pay the pensions, regardless of whether they achieve those returns or not.
但是这是高度可疑的假设，不论政府机构是否能从养老金资产中得到预期的收益率，他们仍需支付这些养老金。
doubtful（adj）（通常作表语）（指人）感到可疑，不能相信；令人生疑的，不肯定的
The weather looks rather doubtful。
天气看起来靠不住。
The fish is also safe, although consumers are doubtful.
尽管消费者有怀疑，但是鱼也是安全的。
Police said he died after playing hide-and-seek with inmates, but this conclusion seemed doubtful.
警察说，他在与室友玩捉迷藏后死亡，但是这个结论似乎是令人生疑的。
But many of his best forecasts, he said, have come from predicting continued progress in China when others were most doubtful.
但他说，自己很多最准确的预测，都是在其他人最怀疑的时候预言中国将继续取得进步。