An evil man is always doubtful.
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Example sentence - It is doubtful the student will graduate as he does not have the credits required.
It is doubtful that it will rain today but take your umbrella just in case.
there is no doubt that scientists will create a cure for most illnesses in the next 100 years
It is doubtful that the Fourth of July picnic will be cancelled due to snow.
I am not writing a sentence using that word.
A synonym for unlikely is doubtful. Now when you are creating a sentence such as; It is unlikely you will remember to do your homework. Replace unlikely with doubtful now you have, I am doubtful that you will remember to do your homework. This should help you understand the meaning of unlikely.
Assuming that you mean doubtful, and example sentence is:It is doubtful that I will live to see the end of all wars.
Example sentence - It is doubtful she will ever like her husbands daughters.
It is doubtful that this sentence will really answer your question, but it is a good start!
a sentence using the word endotracheal
This is a sentence using the word aviator.