Question

when引导的时间状语从句，当表示一个动作发生时另一个动作也发生，前后哪个用一般过去时，哪个用过去进行时

Answer 1

其实按你说的意思，前后两句都用过去进行时也是没有问题的
比如when the boys were playing basketball on the playground, the girls were playing volleyball.
但是按你说的非要一句用一般过去一句用过去进行的话，那就是when后面跟着的用过去进行时，比如：when he was stepping into the classroom, he found nobody was there.当他走进教室时，没人在那儿。强调的是走进教室这个动作。

Answer 2

你说的句型应该是Hardly。。。when。。。几乎未来得及就…；刚一…就…
在该句型中Hardly 后面用过去完成时（不是过去进行时），when 后用过去时
eg. Hardly had I left when she came to visit me. 时间状语从句；时间状语从句我刚离开她就来拜访我了。
Hardly when 也可以连用 eg. I said smilingly to my husband in a soft voice hardly when I opened the door. 给先生开门的时候，我微笑着用温柔的声音对先生说道。

Answer 3

I was doing my homework when my mother came back

前面的用进行时态，后面的用过去时态