What is the subject-verb agreement here?

June 17, 2021 9:00 PM

I want to know the grammatical reasoning behind this: a) Each cat has a collar b) Each cat must have a collar Why does adding must change the subject-verb agreement? This is next to impossible to google an answer for as I'm not sure what grammatical rule/exception I'm looking for, and it is slowly driving me mad not knowing why.
posted by dorothyisunderwood to Writing & Language (13 answers total)

Best answer: "must have" is essentially a contraction of a conjugated verb and an infinitive form.

It's short for "must (to) have." Here, "to have" is the infinitive.
posted by yellowcandy at 9:03 PM on June 17, 2021 [1 favorite]

It falls into the category of auxiliary, or helper verbs. There's only a few of them, and they have their own special rules.
posted by dum spiro spero at 9:04 PM on June 17, 2021 [5 favorites]

If it helps, think of it as
"needs to have a collar"
and then substitute "must" for "needs to".
posted by M. at 9:17 PM on June 17, 2021 [6 favorites]

Because must is a modal verb. Modals are followed by the base form of a verb. This one indicates a requirement. Need to and has to (or have to) express the same idea, but need and have are regular verbs which must be followed by an infinitive.
posted by Rash at 9:21 PM on June 17, 2021 [30 favorites]

Best answer: If you select a different verb, it becomes clearer that the second example is using the infinitive:

1.) Each cat is calico.
2.) Each cat must be calico.
posted by Blue Jello Elf at 9:23 PM on June 17, 2021 [6 favorites]

Put another way, statement A is stating a fact about the world at present (the cats you see before you are wearing collars), whereas statement B is expressing a desired state (ideally, the cats are wearing collars — they may in fact not be). These sentences aren’t the same tense/aspect, so the verbs are conjugated differently.
posted by iamkimiam at 9:34 PM on June 17, 2021 [3 favorites]

Best answer: It's not tense (~time the action took/takes/will take place) or aspect (~extension of action over time) that differs between the sentences, it's modality (~obligatoriness or possibility of action).

In fact, you're still looking at the third person simple present singular form of the verb in both sentences; it's just that "must," a defective verb, is conjugated the same way across the present singular (I must/you must/he-she-it must/we must/they must). And then, as discussed above, as a modal verb, takes the bare infinitive of "have" to follow.
posted by praemunire at 9:55 PM on June 17, 2021 [14 favorites]

praemunire, good clarification — you’re totally correct, I was being lazy and didn’t want to get in the weeds about TAM.
posted by iamkimiam at 11:40 PM on June 17, 2021 [1 favorite]

Another way to think about it is that when you have multiple verbs working together, only the first verb is conjugated. So:

He goes - "go" is the first (and only) verb, so it gets conjugated (3rd person singular present simple, in this case)

He doesn't go - "do" is the first verb here (a helping verb), so it gets the 3rd person sing. conjugation, and the second verb "go" (the main verb) doesn't: it gets the "base form" or "dictionary form" - the thing you have after "to" in the infinitive

He must go - "must" is the first "verb" here, but it's a modal verb, so it has its own rules for conjugation: its 3rd person sing. conjugation is also just "must", without the "s". Same with other modals like will, would, can, could, shall, should, may, might ....
i

Similarly with tense:

I go - the first (and only) verb gets conjugated for 1st person singular present simple.

I went - the first (and only) verb gets conjugated for 1st person singular past simple.

I did go - the first (helping) verb ("do") gets the 1st person singular past simple, while the second (main) verb "go" doesn't get any conjugation - it's just in its base form

I could go - same thing
posted by trig at 12:40 AM on June 18, 2021 [2 favorites]

The same thing happens in German (and I'm guessing other similar languages), but it's less odd, because the "must" changes with the noun. I must, he "musts", kind of change. Just in English, none of those modal verbs change with the noun.

Also in German the second verb is pushed to the end of the sentence, like all* second verbs in German. (*I think). "Each cat has a collar" becomes "Each cat musts a collar have".
posted by kjs4 at 12:53 AM on June 18, 2021 [2 favorites]

Why does "Each cat ought to wear a collar" get the "to"?
posted by clew at 2:18 PM on June 18, 2021 [1 favorite]

Best answer: clew, I think "ought" (in American English, at least) is its own special case, and usually gets the "to"; but one ought not think that means "to" is always necessary :) This page features many examples in British English that sound.... whatever the cool grammarian phrase for "kludgy as hell" is.... to my ears.

Smarter people than me put more thought into these responses, if that's helpful.

Calvin, the late 20th c. philosopher and philologist, pithily summed this up as "verbing weirds language."
posted by adekllny at 2:43 PM on June 18, 2021

It shouldn't surprise you that words that execute very basic functions in a language are irregular in form or function. The more "fundamental" and heavily used, the more likely irregularities are to be preserved.
posted by praemunire at 9:36 PM on June 18, 2021