□「動詞+前置詞」「形容詞+前置詞」が成句(熟語)の場合、「前置詞+関係代名詞」の形にはできない。
⚪︎Is this the key which you are looking for?
これがあなたが探していた鍵ですか。
× Is this the key for which you are looking?
※ look for「探す」が成句
⚪︎I know things which she is good at.
彼女が得意なことを知っています。
× I know things at which she is good.
※ be good at「得意だ」が成句
□関係副詞は「前置詞+関係代名詞」に書き換えられる
This is the town where I was born.
↓
This is the town in which I was born.
ここが私が生まれた町です。
Today is the day when she was born.
↓
Today is the day on which she was born.
今日は彼女が生まれた日だ。
