When did he leave the classroom?— He left ----- you turned back to write on the blackboard.空中可以写alter吗?

When did he leave the classroom?— He left ----- you turned back to write on the blackboard.
空中可以写alter吗?

可以的,但最好注意时态区别,after you had turned back

可以的
注意是after
有不明白的地方再问哟，祝你学习进步，更上一层楼！ (*^__^*