[on Christmas Day 1995]
>What I need is a one-line, easy to memorize rule on using THAT and WHICH.
>I'm never certain which is right.  For example, is it:  

> This is the rule that should clear up the problem.
>                  ^^^^
>                   OR is it:

> This is the rule which should clear up the problem.
>                  ^^^^^
>OR are these two troublesome words essentially interchangeable in 
>the specific use I have illustrated? 

>If somebody will teach me, I promise to permanently implant the correct 
>rule in my eager but confused brain.
Though opinions vary (and you'll get varying opinions in this auspicious group), I'd opine that either that or which would do, in the specific use you have illustrated, which is called a restrictive relative clause.

Alas, there is no possible correct 1-line rule, because both that and which have other uses in other constructions, and other words also occur in all of these constructions. I'll attempt, though, to set out a succinct set of guidelines, but you needn't implant them. Implants are notorious for falling out when you need them.

There are two kinds of grammatical words we need to refer to:

Those are the basic uses of each set of words (call'em the Wh-words and the Th-words). There are other uses as well. In particular, in relative clauses.

There are also two kinds of relative clauses:

  1. Restrictive, which restrict the nouns they modify, by giving some essential property, and which are by far the more common type:
        The man I sent to get the turkey bought a duck instead.
        I intend to fire the man who bought the duck.
        The duck that the man bought cost more than a turkey would have.
        The turkey which we eventually served was overcooked.
    In all of these, the relative clauses (introduced respectively by [ZERO], who, that, and which) pin down the nouns they modify and follow (respectively, man, man, duck, and turkey) and therefore are restrictive. They give necessary information.

    Which and who (occasionally whom, but that's another thread) can be used in restrictive relatives. And so can that. And, if the relative word isn't the subject of the clause, you can also just use [ZERO] if you like. Compare the first one:

            The man who(m) I sent to get the turkey...
            The man that   I sent to get the turkey...
            The man        I sent to get the turkey...
    Since the relative clause has a subject (I) already, the relative word, whatever it is, is unnecessary and may be deleted.

  2. The other type of relative clause is the Non-Restrictive, also called Parenthetic. It gives useful but not necessary information, and is often set off in writing with commas and in speech with intonation dips:
        The turkey, which was overcooked, was nonetheless flavorful.
        I deducted the cost of the duck, which I gave to an orphanage.
    In non-restrictive relative clauses, that MAY NOT be used. If you did use that, you'd have to do without the commas or intonation dip, and you'd convert the clauses to restrictive use:
         The turkey that was overcooked was nonetheless flavorful.
          (implies there was another one that wasn't overcooked)
         I deducted the cost of the duck that I gave to an orphanage.
          (implies there was another, undonated, duck)
Finally, there's another use of that which resembles its relative clause use (because it introduces a clause and because it can be deleted), but is quite different (and is rarely confused with which). That is often used to introduce a finite noun complement, a full sentence that's the Subject or the Object of some verb:
      That the pies burned is perhaps a blessing in disguise.
      I don't think that we're going to have enough stuffing.
Note that you can delete the that in the second but not the first of these, but that you can't use which in either. These aren't relative clauses at all, so which isn't at issue.

Happy holidays.

  - John Lawler       Linguistics Department and Residential College     University of Michigan

    "Language is the most  massive  and  inclusive  art  we know,  a           - Edward Sapir
      mountainous and anonymous work of unconscious generations."       Language (1921)

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