表示“害怕做某事”或“不敢做某事”,be afraid 之后可接不定式,也可接 of doing sth.如：
She was afraid to tell [of telling] you.她害怕告诉你.
She is afraid to go [of going] out alone late at night.她很怕深夜独自外出.
但若要表示担心可能会发生某事,则只能用 be afraid of doing而不能用 be afraid to do.如：
She is afraid of lagging behind the others.她怕落在别人后面.
She didn’t tell him because she was afraid of upsetting him.她没有告诉他,因为怕他不安.