英语状语从句的一点困惑,老师给我讲过如下例句1 he prefers to live where the climate

英语状语从句的一点困惑,
老师给我讲过如下例句
1 he prefers to live where the climate is mild
2 put it back where it had been
3 send it back to where it had belonged
现在感到困惑的是,where前面到底能不能加介词,或者说省掉了介词.也就是
1句能否改成he prefers to live in where the climate is mild
2句能否改成put it back to where it had been
另外,不能加介词的话,2句put it back where it had been中的where it had been 整个做状语我可以理解
如果能加介词,2句 put it back to where it had been中的where做什么成分?无法理解
不应该补上先行词,变成put it back to the place where it had been 是省掉了先行词吗?1句同理