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 do “restrictive” and “nonrestrictive” mean?

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

https://www.thoughtco.com/restrictive-and-nonrestrictive-adjective-clauses-1689689

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 are still confused. It is because the example sentences above are not typical. Let's see:

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

This is why the pen is meaningful.

• I have a pen, which is on the desk now.

It is there now and may be moved somewhere else later.

• I like their program that is open-source.

I mean that open-source one.

• I like their program, which was updated today.

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?

More information

https://jakubmarian.com/is-that-always-restrictive-and-which-non-restrictive/

Other Lessons

• Formation of Adjectives
• Confusion between Adjectives and Adverbs
• Present Tense
• More Uses of the Dash
• Count noun
• Alternate(ly) and alternative(ly)
• CONJUNCTIONS
• Afraid
• Turn an Adjective into an Adverb
• Mass noun