When we pushed the ball it rolled across the floor.
The ball rolled across the floor because we pushed it.
We made the ball roll across the floor by pushing it. Worthy, but wordy. These are fine if you want to focus separately on the pushing and the rolling, but what if you don't? If, for example, you just want to say that the ball-rolling took place, and then move on to more important things (such as why it worked at all)?
As so often, grammar provides just what we need: a causative verb. The verb roll can be used in two ways: The ball rolled across the floor.
We rolled the ball across the floor.In the second example, roll is called a causative verb because it describes the total situation including the causing event (the push).
What's so great about causative verbs? After all, they're ten-a-penny and there's nothing particularly difficult or strange about them. But that's precisely the point: they are all round us, like the air we breathe, and no more strange or difficult than air. But like air, they're rather important and worth some attention.
Are they really that common? Yes, there are several hundred of them, including some of our most common verbs. Take move, for example: The twig moved.
We moved the twig. (= we made it move)
Or wake: He woke at nine.
We woke him at nine. (we made him wake)
Or break: The cup broke.
We broke the cup. (we made it break)
And then there are all the causative verbs that don't share the same form as a non-causative: for example, kill is the causative of ... well, what? And send is the causative of ...?
Why are they so important? Because every time we use one of these verbs we show that we understand causation. If you say that you rolled the ball, you are claiming responsibility for the rolling just as much as if you had said: "The ball rolled because I pushed it." Of course the same is true for connectives like because and so, but we use causative verbs much more often.
Grammar is generous. It provides not one way of describing causes and effects, but many. In some cases the causation is on public display through a special word such as because or make (it happened because I pushed it; my pushing made it happen). But in other cases the cause and effect are both packed away inside a single verb - a marvel of grammatical packaging. As so often, a knowledge of grammar gives us a better understanding of ourselves and the sheer creativity of our linguistic expression.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward V1 School, Bury St Edmunds, Suffolk