adjective
BAD : The road to the airport was very busy and we were afraid to miss the plane.
GOOD : The road to the airport was very busy and we were afraid of missing the plane.
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be afraid to do sth = be unwilling to do something because you are frightened about what may happen: 'She was afraid to eat it in case it was poisonous.' 'Don't be afraid to ask for help.'
be afraid of doing sth = be worried or anxious about something which might happen: 'Most criminals are afraid of being caught.' 'He says that he is afraid of losing his job.'