so early that when he arrived at the city,the gate was still closed.中that和when起什么作用?

（It was）so early that 中的 that 是 so.that 句型的一部分,意思是“如此之早以至于.”
when 是 that 从句中引导状语从句的关系代词.这个从句也可以写成：
the gate was still closed when he arrived at the city（当他抵达这个城市时,城门还未打开）