Language/English/Grammar/That-v.s.-Which

It seems to be both acceptable when it comes to choosing between “that” and “which”. For example,

• I read a book that is about dinosaurs.

• I read a book, which is about dinosaurs.

In fact, “that” leads a “restrictive clause”, while “which” leads a “nonrestrictive clause”.

What does that “restrictive” and “nonrestrictive” mean?

In the first sentence, “that” restricts the book to be about dinosaurs. In the second sentence, “which” just adds a piece of information, not restricting the book to “dinosaurs”.

You may want to ask why it is not “restricted to ‘dinosaurs’” as the piece of information is already spoken out.

In fact, whether the piece of information is important is determined by the speaker. If I just want to say that I read a book, nothing else, I can do so. My saying “which is about dinosaurs” is an extra. But if I intend to let you notice that the book is about dinosaurs, I can do so by using “that” to imply that “the book is about dinosaurs” is important.

It is not surprising that you still can't understand. It is because the example sentences above are not typical. Let's see:

• I have a pen that is from my mother on my birthday.

The pen is precious because it's a gift from my mother on my birthday.

• I have a pen, which is bought 2 months ago.

I remember that someone bought the pen 2 months ago.

• I like their program that is open-source.

I really value open-source.

• I like their program, which is updated today.

I think the program is good and they just updated it today.

• I bought a pair of shoe-pads that are comfortable.

That is the point.

• I bought a pair of shoe-pads, which are black.

Who cares about the color when they are in my shoes?

Impressed now?