1. That is the best restaurant (that) (I) (know).
← That is the best restaurant. ＋ I know it. ※it(代名詞：目的格) ⇒ that(関係代名詞：目的格)

2. I talked with a girl (whose) (dream) is to be a dancer.
←　I talked with a girl. ＋ Her dream is to be a dancer.　※her(代名詞：所有格) ⇒ whose(関係代名詞：所有格)

3. Ms. Mukai is the first (Japanese) (woman) (that) traveled into space.
←　Ms. Mukai is the first Japanese woman. ＋ She traveled into space.　※she(代名詞：主格) ⇒ that(関係代名詞：主格)

4. He is the scientist (whom/who) she (respects) the most.
←　He is the scientist. ＋ She respects him the most.　※him(代名詞：目的格) ⇒ who(m)(関係代名詞：目的格)

参考にしてください。