Let’s imagine this scene: two friends are on vacation and they’ve been having such a great time that they decide to postpone their return to spend a few more days on the beach. A friend back home hears about their decision and says, Them wanting to stay a few more days will be quite expensive. Or should he say, Their wanting to stay a few more days will be quite expensive?
Our question gives us the chance both to answer a specific point of grammar and to illustrate how to proceed methodically in answering such questions when they arise, as they always do, in revision. Let’s begin by remembering that what we’re looking for first in any sentence we take up to review is a clause, and specifically the point of intersection between its subject and its predicate, that is, between what the clause is about and what we’re saying about it. One sentence may have any number of clauses, but our example sentence, I’ll hint, has only one.
Now if we begin at the beginning (which is where we should always begin in reviewing a sentence), the second word, wanting, will appear very much like a verb, and we might well be tempted to conclude that that is the verb of the sentence. But the sentence doesn’t say that someone, some subject, is wanting something; it seems instead to be using the word wanting as a noun, synonymous in some way to the nouns desire or wish. That observation is enough for us to pass over wanting as the verb and continue on step by step through the statement until we come to the phrase will be. Now we can ask ourselves what will be?, and for that question we already have an answer: wanting to stay a few more days.
The subject of the sentence, then, is wanting to stay a few more days, and the predicate is will be quite expensive. Those two halves make up the one clause, the one thought, this sentence intends to communicate. We should now feel some control over the sentence, because seeing this large grammatical frame should give us the confidence to focus on the first question we posed: should it be them wanting or their wanting? We noticed earlier that wanting appears to be a verb but here seems to function as a noun. If we put those two observations together, we could call the word wanting a verbal noun, and that combination of form and function is exactly what grammarians call a gerund.
Gerunds in English always end with the suffix –ing (though not every word that ends with that suffix is a gerund), and they mean to say that not everything in the world around us is a stationary thing; some things, in fact, exist only when they’re moving, even if moving only in our minds. We can speak, then, of a wanting in just the same way that we can speak of a desire or a wish. Desire and wish are nouns but they are not gerunds, because they do not carry any idea of action in their construction. But wanting, with the addition of the suffix –ing, turns a stationary noun into an actional verbal noun—still naming something, though, and so still able, like all nouns, to belong to someone.
And that is the answer to our original question. The pronoun them is used when it means to show the object of a verb or a preposition: I saw them in the park or I gave the letter to them. But their is the form of the pronoun that shows the possessor of something: their vacation or their trip home. And since the gerund wanting is a noun, a verbal noun, it too can be possessed by someone, and so their wanting is the only correct form possible.
And with such close analysis we should have the confidence to press on, no matter the doubt our ear might first have caused.
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My wondering about this has been around for a long time!