when和 while如果从句中两个都是短动作,应该用什么?eg:____he reached the classroo

用when
when引导的句子一般来说都是间歇性动作
e.g.When he reached the classroom,the teacher was having the class.
而你问的问题中的reached the classroom,reach以及后面的come是短暂性动词,自然是用when了
什么时候用while呢
while 一般是引导的是进行时
e.g.My mother was cooking while my father was reading the newspapers.

when he reached the classroom,the teacher came in.
while 只能和延续性动词连用，came是瞬间，所以不能用
ask sb to do是算间歇性动作 不可以持续