我在一本语法书上遇到这样两个句子：
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You need not be afraid of being punished.
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He is afraid to do it.
但是,该书并没有对 afraid of doing 与 afraid to do 这两个结构在语义和用法上进行区别。
afraid of doing 与 afraid to do 这两个结构在语义上是有区别的。afraid of doing 是指“担心、害怕会发生某事”（worried or anxious about sth. which might happen），例如：
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He is not afraid of dying. 他不怕死。
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She seems awfully afraid of losing her job. (LDCE) 她好像特别害怕失去她的工作。
afraid to do 则是指“由于害怕而不愿去做某事”（unwilling to do sth. because you are frightened）。be afraid to do sth. = fear to do sth.（害怕做某事），例如：
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Don't be afraid to ask questions. (ECD) 不要不敢提问题。
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She was afraid to leave her room. 她不敢离开她的房间。
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She was afraid to see you again. (Shaw) 她不敢再见你了。
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He was afraid to take even a drink of water. (Sinclair) 他甚至连水也不敢喝。
[附注]当 be afraid of 后接的动名词表示意志所致的行为时,它也可能表示“(因担心其后果而)不敢去做某事”:
I'm afraid of telling/to tell her the truth. (BEU) 我不敢告诉她真情。