when, while, as, before, after, since, until, as soon as, once, the moment
主从句时态一致:若主句是将来时,从句用一般现在时(“主将从现”)
when某一时间点或时间段
When the bell rings, students stand up.
while 强调动作同时进行,从句动词需延续性(如be doing)
While I was sleeping, my phone rang.
as(伴随性动作或“一边…一边…”)
As spring comes, flowers bloom.