What were you doing when the UFO arrived?和I was walking down the street when a UFO landed down.

题目

What were you doing when the UFO arrived?和I was walking down the street when a UFO landed down.
为什么前者是时间状语从句,而后者是并列句,我不太明白希望各位高手给我指点迷津,谢谢了

答案

前句是有特定的时间限制
后句是强调两个动作同时发生,是并列句