doubtful：是指你对某件事存有怀疑,比如你不相信他说的到底是不是真的
例如：i'm doubtful with his explanation.
suspicious:多半是指怀疑他犯罪.
例如：the police sentenced that he's suspicious.
ambiguous:多半是指事情不太明朗,比较暧昧.
例如：the result of that affair is not very ambiguous.
equivocal:模棱两可的,多半指答案鉴于两个选择之间.
例如：the definition of this word is equivocal.