Now have another look at something Trystan said:
On Tuesdays, we do “air studies”, which is the theory of flying.
This has the same meaning as:
On Tuesdays, we do “air studies”. That’s the theory of flying.
Use “which” to give extra information or to add a comment in a sentence. When writing, use a comma before “which” when what follows is not essential to understating the sentence.
Exercise 2 Join each of the pairs of sentences below with “which”. A. We wear a uniform. It’s blue and with a beret. B. Last Thursday, we did some shooting. It was fun. C. I’ve been up in a Chinook. It’s a military helicopter. D. I want to work in finance. It’s a good career.