Grammar question - subject/verb agreement

[Kathleen in VA]

Two sentences. Same subject; different verbs. Why?

 

She knows how to cook

 

She must know how to cook.

 

Why does adding the word "must" change the verb from "knows" to "know"? Btw, what part of speech is "must"? I'm thinking adverb, but I'm not sure. Anyway, what's up with this?

[Mrs Mungo]

Must is a modal auxiliary verb. It is the modal aspect that changes the tense.

 

Does this help?

 

Or maybe this one?

 

Auxiliary Verbs

The modal verbs have only one form, so subject-verb agreement does not apply to them:

 

eta: Here is a good link on the topic.

 

The grammatical defectiveness of the English modal auxiliaries is well known. In

standard usage they have no infinitive, no present or past participle, and no ‘-(e)s’

inflection with 3rd person singular subjects. The descriptive facts are well established,

but the explanatory question is a theoretical challenge. This is the topic to be

addressed here on a strictly synchronic basis. It outlines a hypothesis about the nature

of the modals that will bring together under the same explanatory concept the wellknown

facts about gaps in the inflectional paradigm. The proposal is that the English

modals can be regarded as inherently non-indicative verb forms in their lexicon

representation. This hypothesis seems to offer an interesting explanatory perspective

on several aspects of modal auxiliary behaviour.

[Kathleen in VA]

I've never heard of modal auxiliary verbs before. This is really quite fascinating. Thanks for the info and links, Mrs. Mungo.:)

[Laurie4b]

Two sentences. Same subject; different verbs. Why?

 

She knows how to cook

 

She must know how to cook.

 

Why does adding the word "must" change the verb from "knows" to "know"? Btw, what part of speech is "must"? I'm thinking adverb, but I'm not sure. Anyway, what's up with this?

 

Must is a helping verb, which is why the main verb changes. It would be the same in the case of other helping verbs: She can know; she will know; she would know, she does know etc.

 

in addition to the "to be" verb forms, other helping verbs are do, does, did, have, has, had, might, must, may, can, could, would, will, should, shall.