Some boys walked so fast that the girls couldn'tkeep up with

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。