be afraid to do　不敢，膽怯去做某事，是主觀上的原因不去做，意為"怕"；
be afraid of doing　擔(dān)心出現(xiàn)doing的狀況、結(jié)果。 doing 是客觀上造成的，意為"生怕，恐怕"。

she was afraid to step further in grass because she was afraid of being bitten by a snake.
她生怕被蛇咬著，而不敢在草叢中再走一步。
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。