be afraid to do：不敢，胆怯去做某事，是主观上的原因不去做，意为"怕"；

be afraid of doing：担心出现doing的状况、结果。 doing 是客观上造成的，意为"生怕，恐怕"。

例句：

She was afraid to step further in grass because she was afraid of being bitten by a snake.

她生怕被蛇咬着，而不敢在草丛中再走一步。

She was afraid to wake her husband.

她不敢去叫醒她丈夫。

She was afraid of waking her husband.

她生怕吵醒她丈夫。