ref – https://www.grammar-monster.com/glossary/relative_pronouns.htm

Relative Pronoun – a pronoun that heads an adjective clause.

• that

  [The dog (that) stole the pie] is back.

  Restrictive Clauses (Essential Information):
  [The book that is on the table] is mine.

  Referring to Things:

  Things: The car that I bought broke down.

  After Superlatives, “All,” “Any,” “Every,” “No”:

  All that nonsense is a waste of time.
  Every book that I’ve borrowed turned out to be gold.
  This is the best movie that I’ve ever seen.
  All that glitters is not gold.
  She can solve any problem that comes her way.
  We’ll destroy any hoodlum that looks at us.

  When the Clause is the Object of the Sentence:

  The cake that she baked was delicious.
  (“That” can sometimes be omitted: The cake she baked was delicious.)

• which

  • My new dog, (which) I bought last year, loves green beans.
  • I rode my bike, (which) now had two bent spokes, back home.

  When to Use “Which”:

  Non-Restrictive Clauses (Extra Information):

  Definition: Clauses that add extra details about the noun but are not necessary to identify it. If removed, the sentence’s meaning remains clear.

  Always set off by commas.

  Examples:

  My laptop, which I bought last year, is already outdated.
  (Adds extra info about the laptop; the clause is non-essential.)

  The park, which has a beautiful lake, is my favorite spot.
  (Provides additional detail about the park.)

  After Prepositions:

  When the relative clause includes a preposition, “which” is preferred.

  The house, in which I grew up, was sold last year.

  The box, for which I paid $20, is now broken.

• who

  • [The person (who) bought his car] found gold under the seat.
  • [The girl (who) stole your phone] is outside.
  • Mrs Miggins, (who) owns a pie shop, is outside.

• whom

  Our lawyer, (whom) we employed for over a year, was related to the complainant.
  

• The choice between “who” and “whom” depends on the grammatical role of the relative pronoun in the clause.

  Here’s a simple guide to help you decide:

  1. Use “Who” as the Subject
  “Who” is used when the relative pronoun is the subject of the clause (the one performing the action).

  Test: Replace “who” with “he,” “she,” or “they” to see if it makes sense.

  Examples:

  The woman (who/whom?) helped me is a doctor.
  (Test: (She) helped me. (She) is at the subject position, so use “who.”)

  The student who won the prize is my friend.
  (Test: He/She won the prize. (He/She) is at the subj position, so use “who.”)

  2. Use “Whom” as the Object
  “Whom” is used when the relative pronoun is the object of the clause (the one receiving the action).

  Test: Replace “whom” with “him,” “her,” or “them” to see if it makes sense.

  Examples:

  The man (who/whom?) I met is a teacher.
  (Test: I met [him]. [him] is at the obj position, so use “whom.”)

  The teacher (who/whom?) we admire is retiring.
  (Test: We admire [her]. [her] is at the obj position, so use “whom.”)

  3. After Prepositions
  Use “whom” when the relative pronoun follows a preposition.

  Examples:

  The friend to whom I spoke is coming over.
  (Test: I spoke to [him/her]. → at obj position, so use “whom.”)

  The candidate for whom I voted won the election.
  (Test: I voted for [him/her]. → at obj position, so use “whom.”)

  whose

  When something belongs to the noun, we use ‘whose’.

  The young girl (whose) cat scratched our sofa has offered to replace the cushions.
  

1. Use “that” for Restrictive Clauses

Purpose: Restrictive clauses provide essential information that defines or specifies the noun. Without them, the sentence’s meaning changes.

No commas are used.

Examples:

The book that is on the table is mine.
(Specifies which book; the clause is necessary to identify it.)

The car that I bought last week broke down.
(Identifies the specific car in question.)

2. Use “which” for Non-Restrictive Clauses

Purpose: Non-restrictive clauses add extra, non-essential information about the noun. The sentence still makes sense if the clause is removed.

Always set off by commas.

Examples:

My laptop, which I bought in 2020, still works perfectly.
(Adds extra detail about the laptop, but isn’t necessary to identify it.)

The park, which has a lake, is my favorite spot.
(Provides additional information about the park.)