Which vs That

OK. What's the difference between WHICH and THAT?...let's take a deep breath and then begin...

Perhaps the first step should be to look at DEFINING RELATIVE CLAUSES. In Cassell's Students' English Grammar, Jake Allsop defines relative clauses as being "like adjectives in that they give information about a noun or a pronoun". However, unlike most adjectives, relative clauses FOLLOW the noun (e.g. the woman that I love, the book that I read).

There are two types of relative clause - DEFINING and NON-DEFINING. Suppose I was to say "I hate the student". Would you know which student I was referring to? I think not, because I have lots of different students. If I DEFINE which student I mean, you will understand more clearly. So, how about "I hate the student WHO ASKS ME QUESTIONS ABOUT GRAMMAR AT THE BEGINNING OF CLASS, BEFORE HE HAS EVEN SAID HELLO TO ME". With this extra information, you can understand that "the student" is Yasir. The clause "WHO ASKS ME QUESTIONS ABOUT GRAMMAR AT THE BEGINNING OF CLASS, BEFORE HE HAS EVEN SAID HELLO TO ME" provides a definition of "the student". OK? Hmm....perhaps I should have chosen a shorter and more simple example. How about "I never eat food". It's not true, is it? So we add "I never eat food WHICH HAS BEEN REHEATED IN A MICROWAVE". The extra information defines the type of food to which I am referring (it's very hard to define defining relative clauses without using defining relative clauses. I love grammar).

The examples I have given so far contain examples of WHO and WHICH, but not THAT. So here's the first key difference between WHICH and THAT:

WHO refers to people (like THE STUDENT WHO ASKS ME GRAMMAR QUESTIONS). WHICH refers to things (like FOOD WHICH HAS BEEN REHEATED IN A MICROWAVE). THAT, however, can refer to both. So we can say THE STUDENT THAT ASKS ME GRAMMAR QUESTIONS and THE FOOD THAT HAS BEEN REHEATED IN A MICROWAVE. Easy. Lovely. Perfect...WHICH = THINGS, while THAT = THINGS or PEOPLE...simple...

If only life were always that simple...actually there are some relative clauses in which it is NOTpossible to use THAT. These are NON-DEFINING RELATIVE CLAUSES, which just give extra information. That last sentence is an example of one, but here are two more:

- I hate Yasir, who comes from Saudi Arabia.

- I read War and Peace, which many people find difficult boring, in less than a week.

In these two sentences, I expect my reader to know who / what I am referring to when I write YASIR or WAR AND PEACE. My reader doesn't need a DEFINITION of these nouns, but I want to provide some extra information, so I provide it through a relative clause. Notice, however, that these relative clauses are surrounded by commas. The comma shows that the clause is separate and, if you like, not ESSENTIAL for understanding my main point. In cases like this, it is NOT POSSIBLE to use THAT. So we can't say "I read War and Peace, that many people find difficult..." or ""I hate Yasir, that comes from...". No no no...

So here is a second key point about WHICH and THAT. Don't use THAT after a comma. This is useful to know, because sometimes WHICH is used to continue a sentence and give extra information about a whole clause or statement. THAT is not used for this person. For example:

- I missed my bus, which is why I was late for class.

- I ate too much cheese last night, which is why I had nightmares.

Remember that THAT does not follow a comma, so if we wanted to use it in these examples, we would have to start a new sentence:

- I missed my bus. That is why I was late for class.

- I ate too much cheese last night. That is why I had nightmares.

Of course, if we are developing an argument in academic writing, we are more likely to use THIS than THAT, because THIS makes the subject seem more immediate and relevant to our reader. This is why (can you see what I'm doing?) you will not find many sentences which begin with THAT in academic writing.

No doubt you are bored of reading this by now, but I should mention two further functions of THAT and WHICH that / which (ha ha) show how different they are.

WHICH is used for questions when, as Jake Allsop states "you wish to identify one or more things or people from a group of things or people". For example:

- Which do you prefer, coke or pepsi?

- Which Spice Girl would you most like to kill...sorry, I mean kiss?

- Which shirt shall I wear tonight?

In such questions, it would be impossible to replace WHICH with THAT. The same is true when we creat a noun phrase with WHICH, such as in the sentence "I can't decide which shirt to wear tonight" or "I don't care which Spice Girl you kill first". Yes, I meant "kill" that time...

Finally, THAT has functions which / that (ha ha again) cannot be performed by WHICH. One is DEMONSTRATIVE, like in the phrase "that time" (two sentences back - remember?), "I want that one!" or even "I don't want to kiss that Spice Girl".

THAT is also a conjuction (e.g."It's so funny that you want to kill the Spice Girls" and "I find it surprising that the Spice Girls are all still alive") and can be used to report speech and ideas - something that university students SHOULD find incredibly useful. Consider expressions like:

- I think that I love you.

- I believe that I love you.

- I know that I love you.

- Don't forget that I love you...

- Have you heard that the Spice Girls have broken up?

...and so on and so on. That's enough love for one evening. Notice, though, how THAT is used to introduce a statement or idea, and it allows us to use one subject and verb after another. I KNOW THAT THIS IS NOT EASY BUT TRY TO REMEMBER THAT I AM DOING THE BEST I CAN...

If you're really clever, you'll ask why I didn't write "the best THAT I can"...but that's a question for another day...