関係代名詞の who と which の使い分けだけに限定して説明します。
英文のどこを見ればすぐにわかりますか？ ⇒ 関係代名詞の前(人か物)と後ろ(動詞か人・物か主語か)を見ればすぐわかります。
以下の例文を見てください。
1.～3.は人＋who/whose/who(m) の例文
1. I have two good friends. [They] live in my neighborhood. → I have two good friends(人) [who](S) live(V) in my neighborhood.
※ 人＋who(主格)＋動詞
2. I have a girlfriend. [Her] father is a high school teacher. → I have a girlfriend(人) [whose] father is a high school teacher.
※ 人＋whose(所有格)＋人・物＋(主語)＋動詞
3. I have two friend. I have known [them] for five years. → I have two friends(人) [who(m)] I have known for five years.
※ 人＋who(m)＋主語＋動詞
4.～6.は物＋which/whose/which の例文
4. I have a great novel. [It] was written by a famous writer. → I have a great book(物) [which] was written by a famous writer.
※ 物＋which(主格)＋動詞
5. I have a great novel. [Its] story is based on real incidents. → I have a great novel(物) [whose] story is based on real incidents.
※ 物＋whose(所有格)＋人・物＋(主語)＋動詞
6. This is a great novel. My friend Ken gave [it] to me. → This is a great novel(物) [which] my friend Ken gave to me.
※ 物＋which(目的格)＋主語＋動詞
1.2.3.の関係代名詞の前には、必ず人がある。4.5.6.の関係代名詞の前には、必ず物がある。
1.と3.(主格)の後ろには必ず動詞がある、2.と4.(所有格)の後ろには必ず人・物がある、3.と6.(目的格)の後ろには必ず主語＋動詞がある。
これが使い分けを知る時に必要な『見る目』です。
いかがですか？
参考になれば幸いです。
You're welcome!
細かい解説ありがとうござます