When I say 'that', I am of course speaking only of 'that' when it is used as a conjunction. It is one of those words that may take form as many different parts of speech, and in that sense, it can be viewed as many different words, and a discussion of them all is outside the scope of today's article. We will be dealing only with 'that' in its conjunctive form, the form in which it is often confused with 'which'.
The difference between 'that' and 'which' is a subtle one, for both are used to introduce a secondary clause. The difficulty is increased by the fact that most people are rather careless about the difference in casual speech. The difference is, however, a fundamental one, and is worth exploring.
Look at the LOLCAT above. I made it specially for you! In the sentence at the bottom, we see 'that' in a typical conjunctive use. The base sentence - 'He's the one' - is modified by the secondary phrase 'that won the championships'. This phrase modifies the object of the base sentence.
Let's look at this in more detail. Suppose, for example, that I were to say to my personal assistant, 'Bring me all the books that have red covers.' I am telling him to make a selection out of the available books, and bring me only those with red covers. I think that's pretty clear, isn't it? In an ideal world, such a request would result in the delivery to me of all the red-covered books in the house. Of course, in the real world, if I asked my assistant to do this I wouldn't get jack, because he has an Attitude Problem. It's hard to get good staff nowadays.
|This is my assistant responding to a polite request.|
You see, what I am doing when I add that secondary clause is qualifying the object of my request. I am limiting its scope - restricting its application to only those books with red covers. If I'd said 'Bring me all the books,' I'd have been asking for every book in the house - or potentially, depending on the context, every book in the world.
Suppose I was asking for a particular book. I might say, 'Bring me Principles of Criminal Law - it's the one that has a picture of a fingerprint on its spine.' (For I'm sorry to say my assistant cannot read. It's a sore trial for a writer.) Hey, I'll still get jack. But at least he knows what he's not bringing me. In my request, I have narrowed the scope of what I want to the one book in my house with a picture of a fingerprint on the spine.
From this, we can derive a rule for 'that' as a conjunction. It is used to introduce a restrictive qualifier. Such a qualifier is generally a phrase, but can be a single word. E.G. - 'Throw all the refugees overboard and shoot the ones that float.' Of course, we hope our Prime Minister didn't really say that. But if he did, he'd be authorising the shooting of only some of his victims. A thrifty use of government assets, wasting bullets only on those who will not drown.
Which, on the other hand, adds meaning to its object. Consider the LOLCAT above. Okay, it is a hound, not a cat, but they are all LOLCATS to me. The base sentence is 'Fifi chose the glass on the left'. The secondary phrase, 'which had more bubbles', does not narrow the scope in any way, but rather extends the meaning attaching to the object. As well as knowing which glass Fifi chose, we now know that her glass had more bubbles than the other one.
Similarly, I could say to Ferret, my assistant, 'Bring me I'm Not Racist But...., which is by a man whose name I cannot pronounce.' The secondary phrase, starting with 'which', does tell us something about the book, although it adds nothing to Ferret's ability to choose it out of all the other books in my bookshelf.
It is tempting to draw an arithmetical analogy, and to say that 'that' is subtractive and 'which' restrictive, but this would not be entirely correct, as 'that' restricts scope while 'which' adds meaning. However, the basic spirit of the analogy makes a convenient hook on which to hang the detailed information.