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".
例如:
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.