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Can we use short answers as a proving method for modal verbs? The rule regarding the short answer to affirmative or negative statements is to repeat the auxiliary verb. eg:
"I can fly."
"So can I."

"I shouldn't stop eating."
"Neither should I."

I was taught that "Have to" is not a Modal Auxiliary Verb because it fails this test. eg:

"I have to run."
"So do I."

But what about Must?
"I must be getting old."
"So must I."

"I must be crazy!"
"So must I!"

Is that really correct? It seems strange to me, but I'd accept it if someone could show evidence to support it. My inclination is to find ways around saying this, but maybe that's out of fear of sounding like a character from Lord of the Rings. I guess this is where "me too" comes in to save the day.

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    But (for me at least, perhaps because I'm British, and getting on a bit), So have I is a perfectly acceptable response to I have to go now. In any case, I think your "rule" for identifying auxiliary verbs is really just a "rule of thumb" (it's probably not a definitive rule that will give the "right" classification in every case).
    – FumbleFingers
    Jan 16, 2020 at 16:55
  • Note that from the linguist's point of view, to have to (be obliged to, usually pronounced HAFF) is effectively a different "word" to the one used as an auxiliary to create Perfect verb forms and so on. Also from the (subconscious) perspective of many native speakers - that's why we pronounce it differently.
    – FumbleFingers
    Jan 16, 2020 at 16:59
  • Of course it's an auxiliary verb. What else could it possibly be? Hint: auxiliary verbs are those with the NICE properties. Check the 'Net for tons of info on this, e.g. link
    – BillJ
    Jan 16, 2020 at 19:31
  • Further: when "have" occurs in clauses expressing obligation usage can be divided. Most speakers treat it as a lexical verb, but some treat it as as auxiliary, cf. "Have I to sign both forms?" (aux) vs "Do I have to sign both forms?" (lexical).
    – BillJ
    Jan 16, 2020 at 19:40
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    So must I could not be more correct. Just like might: I might go tomorrow. So might he. And may. He may leave early tonight. So may I. Careful with mustn't: He mustn't find out. Neither must they.
    – Lambie
    Jan 16, 2020 at 20:59

2 Answers 2

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In short, yes.

An auxiliary verb (abbreviated aux) is a verb that adds functional or grammatical meaning to the clause in which it appears, so as to express tense, aspect, modality, voice, emphasis, etc. Auxiliary verbs usually accompany a main verb.

source: wiki

"must" has 2 functions: deontic modality

You must not mock me.

and epistemic modality

It must have rained.

as for your question,

"I must be crazy!"

"I must be getting old."

Both correspond to epistemic modality.

So, "must" is an auxiliary verb.

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  • "deontic modality" and "epistemic modality" are probably too much for English learners. I don't even know what they mean as a native speaker!
    – CJ Dennis
    Feb 22, 2020 at 6:42
  • simply speaking, epistemic modality means inference, conclusion.
    – WXJ96163
    Feb 22, 2020 at 8:05
  • I don't think it's commonly taught, @CJDennis, but I find myself referring to the two meanings (deontic and epistemic) of modals frequently in answering questions here.
    – Colin Fine
    Mar 23, 2020 at 22:06
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Auxiliaries are important in spoken conversation because the patterns with them are regularly used in it. They are used instead of repetition of lexical verbs in short answers. The syntax with a modal verb 'must' is governed with the so called the auxiliary pattern. The complexity of using auxiliary pattern of the modal verb 'must' is that some short answers with it needs some change of modal verbs. For example, He must be a brother of this student. No, he can't be.

As for your question, a useful link https://english.stackexchange.com/questions/254021/so-must-i-or-so-do-i-in-place-of-of-course/25403

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    Person 1: He must be this student's brother. Person 2: Yes, he must. Only in the negative does it change. Person 2: No, he can't be.
    – Lambie
    Jan 16, 2020 at 21:13
  • Yes, you are right.
    – kngram
    Jan 16, 2020 at 21:17
  • However there is no prohibition against using "he must not be" per se. See these examples
    – chasly - supports Monica
    Nov 20, 2020 at 14:14

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