English

Distinguish that from which

That and which both introduce subordinate clauses. What's the difference between them? Well, in some countries, the two words are pretty much interchangeable. Inevitably though, alert readers from the United States will angrily announce that you confused the two words again.

In the United States, reserve which for nonessential subordinate clauses, and use that for an essential subordinate clause that the sentence can't live without. For example, the key message in the following sentence is that Python is an interpreted language; the sentence can survive without Guido van Rossum invented:

Python is an interpreted language, which Guido van Rossum invented.

By contrast, the following sentence requires don't involve linear algebra:

Fortran is perfect for mathematical calculations that don't involve linear algebra.

If you read a sentence aloud and hear a pause just before the subordinate clause, then use which. If you don't hear a pause, use that. Go back and read the preceding two example sentences. Do you hear the pause in the first sentence?

Place a comma before which; do not place a comma before that.