当他走进教室时,老师已经开始教学了.By the time he_ _the classroom ,the teacher_ _teaching already
问题描述：
当他走进教室时,老师已经开始教学了.By the time he_ _the classroom ,the teacher_ _teaching already
答
by the time 引导时间状语从句时,从句是过去时,主句用过去完成时
从句是现在时,主句用现在完成时或者将来完成时.
此句子讲述过去的事情,所以答案为：
went into/ entered/came into; had been/had started/had begun 皆可