Before I come on to the “that”/”which” rule, just a reminder that “who” should always be used when referring to people.
- The boy who threw the ball.
- This is the woman who always wears a black shawl.
- THAT should be used to introduce a restrictive clause.
- WHICH should be used to introduce a non-restrictive or parenthetical clause.
A restrictive clause is one which is essential to the meaning of a sentence – if it’s removed, the meaning of the sentence will change. For example:
- Chairs that don’t have cushions are uncomfortable to sit on.
- Card games that involve betting money should not be played in school.
- To our knowledge, it is the only body in the solar system that currently sustains life…
- Chairs, which are found in many places of work, are often uncomfortable to sit on.
- I sat on an uncomfortable chair, which was in my office.