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.