What is the difference between a sentence and a clause? The answer has quite a practical purpose, and can help us immensely in revising our work efficiently. Imagine, for example, that you’ve written this sentence and you’re not quite sure you’ve gotten it right: There’s the woman who everybody thinks is going to be the next president of the corporation. The word who bothers you. Should it be whom? Should it be there at all? How, in other words, do we best think about revising what we’ve written?
Let’s begin with some simple definitions. A clause is a group of words with a subject and a verb, and every clause states a thought. A sentence is a group of words that state at least one thought, and so every sentence will have at least one clause, either expressed or implied. We put our thoughts into clauses, and we shape those thoughts into sentences. A clause is a logical device, and a sentence is a rhetorical, or stylistic, device. So what’s so particularly helpful about those distinctions?
We write and speak to convey to someone else the ideas we have in mind. We want to say, both convincingly and persuasively, how we see things, and that means we have to concern ourselves with both the logic and the rhetoric of what we write and speak. If a clause is really a logical device, then seeing the clauses we have written will give us a way to be clear about our thoughts. And if the sentence itself is really a rhetorical device, then seeing the shape we’ve put our thoughts into gives us a way to control how our readers will be affected by the thoughts we are conveying. To convince is to appeal to someone’s reason; to persuade is to appeal to one’s will or emotion.
So if we return now to the example we began with, we see, of course, merely one sentence, but how many clauses does that one sentence comprise? If we expand the opening contraction there’s into its full form, we discover the verb is, and if we read a little further, we find the subject for that verb, the woman. Thus we have our first clause: there is the woman. The standard word order in English is subject + verb, and so this inverted form, as it’s called, might throw off our analysis at first. Nonetheless we have our first clause of the sentence and we can section off these words as one thought.
The next word, who, is a relative pronoun, and a relative pronoun always initiates a clause of its own. The form who is used when the pronoun stands as the subject of its clause, the form whom when it stands as the object for the verb of its clause. Reading who, then, means it must be the subject of some verb yet to come, and if we keep reading without coming to a conclusion too quickly, we find, in fact, that who is the subject of the verb is going to be. So now we have two clauses: there is the woman and who is going to be. And we can quickly count a third when we see that the two words which separate the subject who from its verb, everybody thinks, constitute another subject and verb combination. Thus the writer of this sentence wanted to convey three thoughts in one sentence.
A common question (and common mistake) is why not whom instead of who? After all, the verb thinks needs an object (everybody thinks something), and whom is the object form of the pronoun. Our analysis of clauses, though, helps us avoid that mistake, because to make who into whom would leave the verb is going to be without a subject, and that would break down the grammar of the sentence entirely. And if our ear objects, we just politely let it object, because we analyze with our eyes, not our ears.
But whether the who should be there at all is indeed a question for our ear, because that is a rhetorical consideration meant to appeal to the flow of our words and the emotion that flow evokes. There’s the woman who everybody thinks or There’s the woman everybody thinks: the choice will depend on the writer’s audience and the manner in which one wishes to affect them—more seriously with a complete grammatical statement, more casually with the omission of a word. All of which is to say, we analyze for logic, decide for style, and effect a sentence both convincing and persuasive.