verb
1
BAD : He asked his parents to help him, but they denied.
GOOD : He asked his parents to help him, but they refused.
BAD : When she denied to wear the uniform, she was dismissed.
GOOD : When she refused to wear the uniform, she was dismissed.
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deny sth = say that it is not true: 'He has been accused of stealing a car, but he denies it.' 'Both companies denied that they had been discharging toxic waste.'
refuse (to do sth) = say that you will not do it: 'Employers are refusing to discuss a pay settlement until the staff return to work.' 'The students were told to leave the building, but they refused.'
2
BAD : She asked him if he had seen a little boy but he denied.
GOOD : She asked him if he had seen a little boy but he said he hadn't.
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You deny an accusation or claim (NOT a question): 'The accused denied both charges.' 'He denied being anywhere near the scene of the crime.'
3
BAD : She accused him of cheating but he denied.
GOOD : She accused him of cheating but he denied it.
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Deny is a transitive verb: 'He denied that he forged the signature.' 'He denied having forged the signature.' 'He denied it.'