Some boys walked so fast that the girls couldn't keep up with them. = Some boys walked too fast for the girls to keep up with. = Some boys walked very/quite fast so that the girls couldn't keep up with them. keep up with可以换写成catch up with; couldn't可以换写成weren't able to; 另:第二种变化中,在fast和so that中间也可以加上逗号. 注:/ 表示可以任意选用其一.