Whether to begin a clause with that or which can be confusing. The rules that decides the issue are straightforward:
- Use that to introduce a restrictive clause (also called an essential clause).
- Use which to introduce a non-restrictive clause (also called a non-essential clause).
A restrictive clause restricts or limits the meaning to one particular person, group, place, or thing.
- This is the best paper that you ever wrote.
- I borrowed a pen from the man who sat beside me.
- A professional athlete is one who competes for money.
- Fortunately, he did not lose the wallet that contained his credit cards.
- A restrictive clause is one that restricts the meaning to one particular person, group, place, or thing.
Restrictive clauses are not set off by commas. Restrictive clauses are so closely connected with the rest of the sentence that they cannot be cut away from it by any form of punctuation, not even commas.
Non-restrictive clauses do not restrict or limit the thought to any particular person or thing. Instead, a non-restrictive clause simply adds another idea or fact what was expressed in the main part of the sentence.
- I am studying Chinese, which I find to be a very difficult subject.
- Sailing, which is his favorite sport, is not available at that resort.
- The neighbor's daughter, who has been away at university for several years, has finally returned.
- We left early in the morning for the cottage, which was two hundred miles away.
- His brother, who was the mayor, was an extrovert and highly sociable.
The non-restrictive clauses could all be omitted from the sentences, and the meaning would still be complete and largely unchanged. They are not essential to the meaning.
Non-restrictive clauses are set off by commas.