The traditional distinction between which and that is:
Which introduces a non-restrictive clause or non-defining clause, i.e. a clause that adds extra information to a sentence .
The bag, which I bought yesterday, broke today.
You can take ‘which I bought yesterday’ out of the sentence and it still makes sense. Such which clauses must be separated from the main idea with commas.
That defines the main idea.
This is the house that Jack built.
Such clauses are called restrictive or defining clauses. You can often delete that in these clauses without any loss of meaning.
This is the house Jack built.
Many grammar experts believe that while which should be used for non-restrictive clauses, that and which can be used interchangeably with restrictive clauses.
This is the house which/that Jack built.
If you follow this interpretation, your choice of whether to use which or that will often depend on the formality of your writing — which sounds more formal than that.
Online grammar programs
If you want to learn more about grammar, register for one of Mary’s online grammar courses at www.onlinewritingtraining.com.au
