【could it be in the valley in(which)we had a pi】

问题补充：

could it be in the valley in(which)we had a picnic(that)you left behind your keys and wallet为什么后面一空要认为是强调句来理解呢?为什么不能用where

答案：

in which =where 前面已经使用in which 代替 the valley了,没有表示地点的词语了,楼主想要用where 放在哪里呢 .既然是强调句,去掉强调部分理解下 ,恢复下句子顺序 变成肯定句

you could left behind your keys and wallet in the valley in(which)we had a picnic.

句子去掉了强调部分 it be that 仍保持句子完整,所以,这是强调句