questionable vs doubtful

questionable and doubtful both are adjectives.

Word	Noun	Adjective	Verb	Adverb
questionable	No	Yes	No	No
doubtful	No	Yes	No	No

questionable (adjective)	doubtful (adjective)
subject to question	open to doubt or suspicion
able to be refuted	fraught with uncertainty or doubt
	unsettled in mind or opinion