Is doubtful a verb or a noun?

Answer 1

Doubtful is an adjective; it describes something that is in doubt (noun) or something you doubt (verb). Example uses:

Adjective: The doubtful weather makes it difficult to decide if we should go today.

Noun: My doubt about his ability was proven wrong by his results.

Verb: We don't have much cake left, I doubt there is enough to serve guests.

You can tell it is not a verb because it is impossible to put the preposition "to" in front of it in order to create an infinitive verb, i.e. "to doubtful", or to use it in a sentence such as "I doubtful something". It also cannot be a noun because you cannot put "the" or "a" in front of it and use it in a sentence like "I see the doubtful" or "Can I have a doubtful?"

"Doubtful", as an adjective, means that it can be used either as an attribute, describing something ("The painting was of doubtful origin") or as a predicate adjective which usually appears alone after a verb ("I am doubtful" or "she is feeling doubtful"). "Doubtful" is probably most commonly used in this second way, as a predicate adjective. As an attribute, other words such as "dubious" (which is related to the word "doubt") are more often used.

Answer 2

"Doubtful" can function as both an adjective and a verb. As an adjective, it describes uncertainty or skepticism. As a verb, it means to feel uncertain or have suspicions about something.