A little knowledge about something can cause a lot of unnecessary confusion, and when the subject is a complex one, as grammar certainly is, we are often impatient in being presented with yet another term making yet another, finer distinction. Not having that little bit more of knowledge, though, can mean the difference between somewhat understanding a subject, which is where frustration is born, and understanding it inside and out. And when we have the right conceptual tool, there can be a distinct pleasure in finally seeing through what was a mist of confusion.
Consider, for example, what difficulty arises if we just ask what the verb is of this workaday sentence: I want to leave soon. That simple question does not have so simple an answer if our analytical toolkit includes only the definition that a verb is an action word. With that simplistic mantra in mind, we are more likely than not to say that leave is the verb, because getting up and going (something you might even be wishing to do right now) is clearly a real action. We wouldn’t be wrong in saying so, but we wouldn’t be entirely right, either—whereupon we come upon a confusion that need not be. With only the dull knife definition that a verb is an action word in hand, we can’t cut much in a sentence even as simple as this one.
Now it is true to say, of course, that verbs indicate action, but what about the word want in our sentence? Is not feeling the need for something an action too, even if that action is underway only in our mind? Certainly so, which means that we can be served well in grammatical analysis by the distinction that actions can be both external and internal. Something acts when it moves, when it changes, and though we might pride ourselves in believing we are always as constant as the northern star, all the incessant change we see in the world is going on in our mind as well, being in one mood now, for example, and another mood later. With the distinction, then, between physical action and mental action, we realize that the simple sentence I want to leave soon has two verbs: want and leave.
To settle with that as an answer, though, would be to settle with an ever wriggling riddle, for how can a clause, which is the combination of a subject and verb, have two verbs? Very easily and often, in fact, and we can solve the riddle and slip the knot when we learn that grammar makes a distinction between a finite and infinite verb. The word finite means specific, and so a finite verb is one that specifies person and tense (among a few other things that need not concern us here). The finite verb of a sentence is the one that works with a subject, the one that carries the primary meaning and around which the rest of the sentence is built. In the sentence I want to leave soon, the verb want works with the subject I, and is therefore the finite verb of the sentence.
When we are under the task, then, of finding the verb of a sentence, what we’re really looking for is the finite verb, not simply any word that expresses action, external or internal. And that little bit more of knowledge, that sharper conceptual tool of a finite verb, makes for a finer distinction that can clear the way through the apparent difficulty of coming up with two verbs in a sentence. This then puts us in a position to understand that the other verb in our sentence, leave, is not a finite verb, for it does not specify a subject and convey the central meaning: I am not leaving, I am wanting. The verb leave, then, is an infinite verb, or what is called an infinitive; it expresses action, certainly, but not the central action that is gearing up the meaning of the sentence. What I want is what the sentence is really about, and what I want is to leave, an infinitive verb that is the object of the finite verb want.
Thus it is that the one-step-further distinction between a finite and infinite verb can loosen apart a bedeviling grammatical question, even one as apparently simple as asking what the verb of a sentence is. All this can be an occasion to remember that stopping short is where the real problem often lies, and with a few finer conceptual tools, what looked like a problem clears away before our sharper analytical eyes.