One of the persisting confusions for writers is the difference between the verbs lie and lay. If it’s been a long day, do I lie down or lay down to rest for a few minutes? There is nothing intrinsically difficult about using these two words if—and it’s an unfashionable condition, I know—we apply a little grammatical theory. Let’s see where it leads.
Verbs in English can be organized into two large groups: transitive and intransitive. The term transitive derives from the Latin meaning goes across; we can see our word transit in the grammatical term, and we call a system of trains and buses public transit because they go across the city. When the notion of going across, or transitivity, is applied to grammar, it points to the idea that in certain verbs, the subject is understood to be doing something that directly affects something other than the subject. That something else, called the direct object, is thought (somewhat imaginatively) to be changed in some way because an action left the subject and went across to the direct object. Thus, a defeated militia lays down its arms, where lays is the transitive verb and arms its direct object.
Intransitive verbs, on the other hand, do not have a direct object (the prefix in– often means, as it does here, not). The action of an intransitive verb is thought to remain within the subject itself, and therefore to change only the subject in some way. If the day’s work has so worn me out that I want to recline, or rest in a horizontal position, for a while, then I lie down for a rest, and the only thing that has moved from a vertical to a horizontal position is I myself, the subject.
So far, so good. Lay is a transitive verb and lie is an intransitive verb. The problem comes in when we change the tense. Both examples we’ve just looked at (a defeated militia lays down its arms and I lie down for a rest) are in the present tense. But what happens if all this took place yesterday? Well, then, the defeated militia laid down its arms and I lay down for a rest. The simple past tense of the transitive verb lay is laid, and the simple past of the intransitive lie is lay. Or, to say it another way, lay is the simple present of the transitive and simple past of the intransitive. Whence the common confusion.
And hence the importance of a little theory. We have grown weary and suspicious as a people (I say this only half facetiously) of understanding the principles that underlie a practice, be it gardening or governing or grammar. We want results, we want to get it done, and so we often rely on what sounds right or seems correct. The problem with that intensely individualistic approach, of course, is that we are relying only on our own experience: I laid down for a rest is correct, we conclude, because that sounds right. But appearing correct is not necessarily being correct. And as dry and unnecessary as the principles can seem, theory objectifies, and thereby gets us free of our own assumptions, to test them and change them if need be. Practice and theory always go hand in hand, theory to guide and practice to accomplish accordingly.