If you are doubtful about something, you are uncertain about it.

The teacher is doubtful about having parents working as classroom assistants. 这位老师对由家长担任课堂助教一事拿不定主意。

If a situation is doubtful, it is unlikely to happen or to be successful.

It is doubtful whether/if they ever reached the summit before they died. 他们死前不太可能到过山顶。

It was doubtful that the money would ever be found again. 再找到那些钱不太可能了。