Why does my teacher say I should use ‘which I bought’ instead of ‘that I bought’?

Question

My new computer, that I bought last week, has already broken down.

My teacher told me ‘that’ is wrong; it should be ‘which’ as it is an object. However, I don't understand why I can't use ‘that I bought’.

Can anyone explain why?

Answer 1

It probably should be "which" but not because it's an object.

This is a non-restrictive relative clause. It describes your computer, it doesn't identify it. I know it is a non-restrictive clause, partly from the meaning (unless you have two new computers, but only one was bought last week), and partly from the use of a comma.

So your new computer is already fully identified by the phrase "my new computer", and the relative clause describes this computer.

Compare that with

My shirt that I bought last week....

Here the relative clause identifies the particular shirt that I am talking about (not my old shirts, but the one I bought last week).

When a relative clause is used to describe you should use a comma, and use the word "which". A simple rule for learners is: if you use a comma, use "which", no comma then use "that".

That is the "rule", but unlike some rules, this one is often broken by native speakers, and there are differences of dialect too.