i haven't found my wallet yet.i don't konw whether it was in

I don't konw whether it was in the taxi ___ I lost it yesterday evening.空格上填 that,这里就是考查强调句.如果把 whether 后面的宾语从句单独提出来看,强调句为：
It was in the taxi that I lost it yesterday evening.
还原为非强调句,则是：I lost it in the taxi yesterday evening.
原题对地点状语 in the taxi 进行强调.
如果你一定要改写成定语从句,那需要把 in 去掉,可以这样说：
It was the taxi in which I lost it （这个 it 指 my wallet）yesterday evening.
希望这个解释能让你看懂.

先看这句话：
i don't konw whether i lost it in the taxi yesterday evening 我不知道是否昨天把它落在出租车上了
很顺畅吧？
这就可以看出，去掉it was that之后并不影响原句的结构和含义，所以应当是强调句。
如果填where或in which，就变成了定语从句，翻译成 我不知道它是否在我昨天把它弄丢了...