1. „That” (Essential Information)

Use that to introduce information you must have to understand the sentence. If you remove the part with „that,” the sentence loses its core meaning or doesn’t make sense.

Rule: Do not use commas before „that”.

Example: „The car that I bought yesterday is red.”

Why? „That I bought yesterday” tells us which car we mean. If you just say „The car is red,” we don’t know which car you are talking about.

2. „Which” (Extra Information)

Use which to add extra details that are not strictly needed. If you remove the part with „which,” the main sentence still makes complete sense.

Rule: Always put a comma before „which”.

Example: „My car, which I bought yesterday, is red.”

Why? We already know it is my car. The fact that I bought it yesterday is just extra information. If you take out that middle part („My car is red”), the sentence still works fine.

3. „What” (The Thing That…)

Use what when it means „the thing(s) that.” It doesn’t connect back to a specific noun in the sentence; instead, it acts as the noun itself.

Rule: Do not put a noun right before „what.”

Example: „I know what you did.” (Meaning: I know the thing that you did.)

Another Example: „This is what I want.” (Meaning: This is the thing that I want.)

Quick Comparison Table

Using „which” in restrictive clauses is , though strict traditional grammar rules prefer „that” for essential information.

4. The Traditional Rule vs. Real-World Usage

a) The Traditional Rule: Strict style guides (like The Chicago Manual of Style) say you should use that for restrictive (essential) clauses and which for non-restrictive (extra detail) clauses.

Restrictive: „The car that broke down is outside.” (Essential to know which car)

Non-restrictive: „My car, which broke down, is outside.” (Extra info, separated by commas)

b) Modern Usage: In actual practice—especially in British English—using which in a restrictive clause (without a comma) is completely normal and widely accepted.

Example: „The house which has the red door is mine.” (Fully correct in common usage, though a strict editor might change it to „that”)

5. “Who”

a)replaces the person not mentioned, known yet

They never found out who the murderer was. She wondered who had sent the flowers.

b) used after a noun to show which person or which people you are talking about

Do you know the people who live over the road? The woman who was driving… She was the one who did most of the talking.

c) used, after a comma in writing, to add more information about a particular person or group of people that you have just mentioned

I discussed it with my brother, who is a lawyer. Alison Jones and her husband David, who live in Hartlepool, are celebrating their golden wedding anniversary.