1. Doubtful about/of

A) For people

If a person is uncertain about something, we can use DOUBTFUL ABOUT/OF or DOUBTFUL. If there is a noun after DOUBTFUL we use DOUBTFUL ABOUT, if there is no noun after we use DOUBTFUL; e.g.

i) We were doubtful about/of the product’s usefulness from the start.
ii) I’m a bit doubtful about/of whether to take on the job, as the hours are pretty unsociable.
iii) Why did he sound so doubtful? (There is no noun after DOUBTFUL, so we haven’t the preposition ABOUT here.

B) Not for people

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

i) It is doubtful that/ whether/if they ever reached the summit before they died.
ii) It was doubtful that/ whether/if the money would ever be found.
iii) It is doubtful that/ whether/if the government will ever agree to their demands.
iv) It now looks doubtful that/ whether/if the building work will be completed on time.

NOTE-I: It was formerly considered correct to use whether after doubtful, but now if and that are also acceptable

NOTE-II: If there is a clause after DOUBTFUL we use THAT, WHETHER or IF, if there is no clause we use only DOUBTFUL; e.g.

a) It was doubtful that/ whether/if the money would ever be found (There is a clause (if the money would ever be found) after DOUBTFUL)
b) Already the whole project was looking doubtful. (There is no clause after DOUBTFUL)

2. Doubtful for

If a sports player is DOUBTFUL FOR a match or event, he or she seems unlikely to play, usually because of injury, e.g.

i) Rohit Sharma is doubtful for tonight’s match.
ii) He is injured and is doubtful for the game tomorrow.

3. Of doubtful quality/value/reliability, etc.

If something is of doubtful quality, value, reliability, etc. we mean that it is of low quality, value or reliability, etc. Sometimes OF is not necessary; e.g.

i) These mangoes are of doubtful quality.
ii) They also seemed of very doubtful value.
iii) Information that he described is having doubtful reliability.

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