A: She is my best friend.
B: She was my classmate in highschool.
これらを関係代名詞でつなげます。
She is my best friend who was my classmate in highschool.
このSheの代わりに物が来る場合は、whichやthatを使います。
A: There is a cafeteria
B: The cafeteria is full of college students.
There is a cafeteria which is full of college students.
(Thatを使ってもOKです)
お礼
ありがとうございます。十分に参考となりました