Which v. That: Non-defining v. Defining clauses

Defining and non-defining relatives pronouns are complicated and even a little scary at first, but I give three fast and dirty rules at the end that you can use, and skip all this explanation. In the previous sentence, I have already used  and underlined a defining relative clause. 

If not, consider the following:

With a lit cigarette, Noah deliberately burst the balloon that the little boy  had got for his birthday.

With a lit cigarette, Anastasia deliberately burst the balloon, which the little  boy  had got for his birthday.

Which of these two students is more evil? I would say Noah.

The reason is that in the first sentence, in which we use that without a comma, the clear implication is that the little boy  got a balloon for his birthday – and nothing else. And then Noah comes along and bursts it.  In other words, the that without a comma is defining: it tells us what the child’s (ex-) birthday present was before nasty  Noah came along.

In the second example, Anastasia has also  burst a child’s balloon, and we all  think very ill of her. She is as bad as Noah in all respects. Except - here is a crumb of comfort -  maybe the child also got other things for his birthday. After all, the effect of the comma and which is to add in some extra information, almost as if we were saying – and, oh, by the way,  he got that balloon for his birthday. But maybe he also got a bear and a puppy and a cake. We simply don’t know, because the which clause is non-defining. It adds information to the balloon, but it does not absolutely single out that balloon as being the one thing the poor boy  got for his birthday.

Similarly:

The Ferrari  that is in my garage is red.

The Ferrari, which is my garage, is red.

With the that clause and no comma, you may well deduce that I, grown fat and rich from teaching, have more than one Ferrari.  After all, the one that is in the garage is red…but the one on my driveway is yellow with go-faster stripes, and the one beside my swimming pool is white with gold trim.  Since the that clause is defining, it wants you to know that we are talking about this particular Ferrari and not that other one, which implies I do have another.

With the which pronoun  with a comma, you will probably  assume that I have just one Ferrari. The car – oh, and by the way it is in the garage, I am saying this by way of extra information (but non-defining) – is red. And it’s in the garage. Just one car. This is a non-defining clause.

One more complication: we may replace that with which but we may not replace which with that. Or, to put it more clearly, if we have no comma, we may use which or that and the clause is defining. If we do have a comma, we may use only which and the clause is non-defining.

Accordingly, we may not write, The computer, that is on the desk, is running Windows 10.

We may, however, write, The computer, which is on the desk, is running Windows 10 [I probably have just one computer] or The computer that is on the desk is running Windows 10 [I probably have more than one computer] or The Computer which is on the desk is running Windows 10 [same as before, I probably have more than one computer].

Quick and dirty rule (1): The comma-which non-defining construction is far more common. Use it, and you will be wrong less often. 

Quick and dirty rule (2): If you can leave out the relative clause entirely and say The computer on the desk is running Windows 10 instead of The computer that is on the desk… then the expression is  defining. In short, if you can leave it out, but want to put it in, you should be using that without a comma.

Quick and dirty rule (3):  Use either  comma-which or  no comma-that. Don’t bother with the no-comma-which construction until you are more confident.