[MrPedantic]

Hello PiP

A defining 'that/which' precedes a clause that limits the meaning of a noun.

A non-defining 'that/which' precedes a clause that gives additional information about a noun or another clause.

For example:

1. 'He doesn't like the hat that you were wearing on Sunday.'

Here, the underlined clause limits the meaning of 'hat' to 'Sunday's hat'.

We therefore learn only one fact: 'he doesn't like your Sunday hat'.

2. 'He doesn't like pizza, which is quite unusual these days.'

Here, the underlined clause gives further information about the fact that he doesn't like pizza.

We therefore learn two facts:

a) He doesn't like pizza;
b) Not to like pizza is unusual.

MrP