When Mr. Ruan entered the classroom, one student was using his cell phone. After Mr. Ruan had left the classroom, one student took out his cell phone immediately. One student took out his cell phone as soon as Mr. Ruan left the classroom.
时间状语
从句
常由从属连词when, while, as, since, till, before, after, by the time 等引导。
Grammar 1 Adverbial clause of time时间状语从句