dubious,suspicious,doubtful和skeptical的区别?

Dubious:中译为“半信半疑”,词组搭配为：dubious about sth/doing sth
e.g.I am still dubious about her motive.我对她的动机仍然存有疑虑.
Suspicious:中译为“表示怀疑的”,词组搭配为：suspicious about/of sth/sb
e.g.I am suspicious of these strangers.我对这些陌生人表示怀疑.
Doubtful:中译为“感到怀疑,不能确定”,词组搭配为：doubtful about sth/doing sth
e.g.She feels doubtful about going to or not.她拿不定主意去不去.
Skeptical:中译为“多疑的”,词组搭配为：skeptical about/of sth
e.g.She feels skeptical of political promises.她对政治上的许诺表示怀疑.
显然,这四个词在搭配词组方面表示怀疑,意思相当接近,有些可以替换.但是还是有一些差别.Dubious强调有一半相信,当它做形容词时有“有争议的”之意.而且也有做贬义,形容一个人名声不好,不可靠,如：His background is a trifle dubious.他的背景不大可靠