from a background briefing which i had prior to departure i
from a background briefing which i had prior to departure i learn that...
书上翻译成:就在出发前,我从一则背景介绍得知.
问:为什么可以这样翻译的?在这里,应该不是一个时间状语从语,前面将的是背景介绍,是一个介宾结构,后面的WHICH指代的是BACKGROUND BRIEFING,该是个宾语从句.怎么就翻译成一个时间状语从句了?能不能分析一下这个句子?