関係代名詞、とくに先行詞が人の場合、用法が主格/所有格/目的格かで形が変わるので注意！
1. She has some friends who are lawyer. （主格）
She has some friends. ＋ They are lawyer.
2. I have a friend whose name is Mary. （所有格）
I have a friend. ＋ Her name is Mary.
3. The old man whom I saw on the street is my sibling. （目的格）
The old man is my sibling. ＋ I saw him on the street.
