1This is the most expensive pen which I have bought.
2I will never forget the day which I first met you on the ship.
3This is all what I know about the mater.
which改成that，因为句子中出现了最高级，引导词只能用that。
which改成when，定语从句中缺少时间状语，所以用when
what改成that，因为what不能引导定语从句。（或者却掉all，也可以，这时候what引导的是宾语从句）
将which改为that。因为先行词是最高级的形式，定语从句只能用that
将which改为when。因为从句中不缺任何成分，when作状语，相当于on which
将what改为that 。因为从that到句末引导的是all的定语从句，当然这里的that也可以去掉，即
This is all I know about the mater.
望采纳，谢谢！