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擔心出現doing的狀況、結果。 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。