(1) I was looking for the man whom I had promised to meet at a café.
(2) The jacket which I bought online didn't fit me well.
(3) I have a lot of email messages which I have to reply to./I have a lot of email messages to which I have to reply.
(4) We would often visit that park, which is famous for beautiful roses.
(5) Mr. Tanaka, who used to be an actor, is a writer now.
(6) Bob said he had never seen that girl before, which was true.
参考にしてください。
訂正：(3) I have a lot of email messages.＋ I have to reply to [it](代名詞・目的格).
→ I have a lot of email messages.＋ I have to reply to [them](代名詞・目的格).
関係代名詞の役割：二つの英文の共通する人・ものを一つにまとめる働き
(1) I was looking for the man.＋ I had promised to meet [him](代名詞・目的格) at a café.
→ I was looking for the man [whom](関係代名詞・目的格) I had promised to meet at a café.
(2) The jacket didn't fit me well.＋ I bought [it](代名詞・目的格) online.
→ The jacket [which](関係代名詞・目的格) I bought online didn't fit me well.
(3) I have a lot of email messages.＋ I have to reply to [it](代名詞・目的格).
→ I have a lot of email messages [which](関係代名詞・目的格) I have to reply to./I have a lot of email messages to which(関係代名詞・目的格) I have to reply.
(4) We would often visit that park, [and](接続詞) [it](代名詞・主格) is famous for beautiful roses.
→ We would often visit that park, [which](関係代名詞・主格) is famous for beautiful roses.
(5) Mr. Tanaka, [though](接続詞) [he](代名詞・主格)] used to be an actor, is a writer now.
→ Mr. Tanaka, [who](関係代名詞・主格) used to be an actor, is a writer now.
(6) Bob said he had never seen that girl before, [and](接続詞) [it](代名詞・主格) was true.
→ Bob said he had never seen that girl before, [which](関係代名詞・主格) was true.
現在使っている英文法参考書の『関係詞』の中にある「関係代名詞」を、ご自分の目で最初から見て、確認してください。
上記のような説明と具体的な例文が必ず書かれています。
是非、手元にある英文参考書を活用してくださいね。