as解释为"当"的时候主从句的时态有限制吗?

一般没有限制,唯一的限制来自语境的需求,
比如
As we are approaching the summit,through the treetops we can see the valley below and ...
As we were approaching the summit,through the treetops we could see the valley below and ...
As we approached the summit,through the treetops we could see the valley below and ...
语法上都对,
第一句现在时态通常用于现场报道,但主宾时态要一致：are approaching ...can see
第二、第三句用于叙述情景.
不可用 approaches ,这是第三人单数,只有前面是 he/she 时可用：
As he approaches ...he can see ...