I doubt whether their journey is comfortable or not.或者I doubt if their journey is comfortable .
顺便说说.
whether和if都可以用来引导疑问句.whether 和if都可以和or not连用,但是whether之后可以直接跟or not,而if则不能,如：I don’t care whether or not your car breaks down.
I doubt whether their journey is comfortable or not.或者I doubt if their journey is comfortable .
顺便说说.
whether和if都可以用来引导疑问句.whether 和if都可以和or not连用,但是whether之后可以直接跟or not,而if则不能,如：I don’t care whether or not your car breaks down.