they should encourage those among us who cannot swim five yards before they are gasping for air.

题目

they should encourage those among us who cannot swim five yards before they are gasping for air.
那个who引导的定语从句是修饰those的吗?另外before引导的时间定语从句为什么要现在进行时呢?

答案

who引导的定语从句是修饰those
现在进行时常表示将要