两个短语都有“不敢去做某事”的意思,但be afraid to do sth.真正的意思是“害怕做某事”,也就是主语对做某事内心感到恐惧；be afraid of doing sth.表示主语内心里不情愿做某事,也可以说是不希望发生某事,但自己不一定能避免它.例如：
He was afraid to go out alone at night.(他害怕晚上独自外出.暗含的意思是“他对于晚上独自外出缺乏安全感”.)
He came late to school and was afraid of being scolded by the teacher.(他上学迟到了,害怕被老师责.
用“不敢.”来辨别这两个短语很能说明问题.
He is afraid of being beaten by his father.他怕被他爸打.（不是他敢不敢的问题,他是不情愿.）
He is afraid to touch the snake.他不敢碰那条蛇.（他内心里恐惧蛇.）