This is the watch for which Tom is looking 怎么改病句

This is the watch at which Tom is looking.
把for改成at
主动形式是Tom is looking at this watch.
所以被动的时候要用at而不是for

This is the watch which Tom is looking for.(原句：Tom is looking for the watch. look for是固定搭配:寻找。)

This is the watch which Tom is looking for.
一般情况下，词组里面的介词，在从句里面一般不前移，不前移一定可以，但是前移就要看情况。就像这个句子，前移了反而是不好的，成了病句。
常见的，有in的词组，表示 在......里面 的意思的时候，可以前移。
仅供参考。

This is the watch which Tom is looking for?

This is the watch( which)Tom is looking for,句子结构属定语从句,which在从句中充当宾语,可省略.