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"。
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。