Peter was about to unlock the door ___ he found someone had broken into the room.
问题描述：
Peter was about to unlock the door ___ he found someone had broken into the room.
为什么要填WHEN,在这里是什么作用?
答
when 在这里是并列连词,意为“其时,就在这个时候”,用来引出一个意想不到的动作（下一分句中的动作）