A friend recently told me that "can" is a rare verb without an infinitive. I have since looked it up and discovered it is an auxiliary verb. In my mind it modifies a "proper" verb in much the same way an adverb does.

"I can jump puddles"

"I could do it"

"to jump" and "to do" are OK, "to can" is wrong (meaning "to be able to").

"I am able to jump puddles"

"I was able to do it"

So, are words like "can/could", "must", "will/would", etc. actually verbs or are they only auxiliary or modal verbs and not "proper" verbs? I was also chatting to a Frenchman and he said in French auxiliary verbs are just called auxiliaries and are not considered to be verbs, so is it the same in English? It seems to me that if you can't "to" or "-ing" a word it is not proper verb.

"to jump", "jumping" - OK

"to must", "musting" - wrong

However, if auxiliary and modal verbs are actually verbs is there a word to describe a non-modal, non-auxiliary verb?

Please, no comments about canning factories or making wills. I already know that the words "can" and "will" can be infinitives but with a different meaning.


Black bears, brown bears and polar bears are actually bears. Koalas and pandas are not bears yet some people call them koala bears and panda bears because they look a bit like bears. Likewise jellyfish and starfish are not fish but are aquatic animals as fish are. Alternative names for them are sea jellies and sea stars. Ironically, the seahorse is actually a fish. Things are not always what they're called. So are auxiliary verbs actually verbs or just called verbs?

  • This is probably not an acceptable contribution to the question, but I want you to know that seeing modal verbs as adverbs is very interesting and makes sense (e.g. 'it might make sense' = 'it make sense' (in the present subjunctive?) + the 'adverb'). I've never seen such a view. Cheers.
    – user100308
    Commented Dec 7, 2014 at 9:07
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    It might make sense - but it probably doesn't. You've only to look at Might it? to see that they need to be treated as a verb or something like a verb. Semantically you can see them as modifying the main verb in the same sort of way as adverbs do, but syntactically it doesn't work. And parts of speech (or grammatical categories) are mostly defined in terms of syntax, not semantics.
    – Colin Fine
    Commented Dec 7, 2014 at 10:51
  • French doesn’t have modals: when you form multi-verb chains in French, you still inflect the first one for person, number, tense, and mood. So for example, "I can jump" and "We can jump" and "We could jump" all inflect the "can" word differently. Furthermore, it still has non-finite forms like infinitives and participles. None of these things is true about English modals, which are formally “defective” in this regard, meaning that they lack those forms. They had, however, a class of verb, and they continue to have like verbs syntactically. Note: English has non-modal auxiliary verbs, too.
    – tchrist
    Commented Dec 7, 2014 at 14:51
  • 1
    @tchrist But most English modals are inflected for the feature-formally-known-as-tense--can/could,may/might, shall/should, will/would--and even must used to be: mote/must. Commented Dec 7, 2014 at 16:29
  • We now know that pandas are indeed bears (and not racoons!) Commented Nov 9, 2019 at 23:24

5 Answers 5


To determine whether auxiliaries are verbs, we should examine two kinds of properties.  One kind of important property relates to word forms, and the other kind of property relates to word use. 

Most verbs have properties such as tense, aspect and mode. 

The verb to be is a normal, complete verb.  Was and were are past tense forms of to be, and is, are and am are present tense forms.  Being is a continuous aspect form, and been is a perfect aspect form.  The were of "if I were a rich man" and the be of "be he live or be he dead, I'll grind his bones into my bread" are subjunctive forms. 

If can is a verb, then it is a defective verb.  It doesn't have forms that show aspect.  The forms canning and canned don't exist for the auxiliary.  Neither does the infinitive to can.*  However, it does have the properties of tense and mode.  Could is both a past tense form and a subjunctive mode form, just as were is both a past tense form and a subjunctive form of to be


Word form isn't the only way to show a verb's mode.  The interrogative mode is usually shown by the verb's position. 

The statement "he is a student" employs the indicative mode.  The interrogative mode places the first word of the verb** before the subject: "is he a student?"  The statement "he can be a student" is subject to the same transformation: "can he be a student?" 

A clause pairs a subject with a predicate.  A predicate requires a verb.  We consider a statement like "he studies" to be a complete clause.  In answer to the question "is he a student?", the answer "he is" also counts as a complete clause.  If, in answer to "can he be a student?", the statement "he can" is a complete clause, then the can must be a verb. 

Although we can see that the auxiliary can doesn't exhibit every property that most verbs have, it does exhibit properties that only verbs have.  It has a form that marks tense or mode.  Its position can indicate a mode that word forms cannot.  It can act on its own as the predicate of a clause. 

All these reasons support the idea that defective verbs are verbs.  We don't have a good reason to place such words in a different grammatical category. 


* Yes, there are homonyms that do have continuous, perfect and infinitive forms.  We won't consider those to be the same verb.

** Most one-word verbs require that a word be added to the verb phrase so that the added word can be moved.  The statement "he studies" becomes "he does study" on its way to becoming "does he study?"  The notable exception to this is when the one-word verb is a form of to be

  • Thank you for being the first person to write an answer to the question I was actually asking! I thought at first that your second ** point was due to the difference between transitive and intransitive verbs but I quickly realised it works with both. "He studies", "He studies English", "Does he study?", "Does he study English?" "Is he a student?" "He is a student", "Is he?" "He is!"
    – CJ Dennis
    Commented Dec 8, 2014 at 2:12
  • 'Although we can see that the auxiliary can doesn't exhibit every property that most verbs have, it does exhibit properties that only verbs have.' begs the question. Commented Mar 7, 2017 at 22:33

The English modal auxiliary verbs

are, indeed, defective verbs in that they no longer inflect for anything and have become part of the machinery. If you want to call them Aux instead of Vb, you're within your rights.

However, then you have the problem of whether be is a verb. It turns out that be is always an auxiliary for some kind of construction: Passive, Progressive, Predicate Adjective, Predicate Noun, and others like

  • He is to leave tomorrow.
  • It's a long way to Tipperary.
  • There's a man looking for you.
  • What I need is love.

Unlike lexical verbs, the behavior of be is completely governed by grammatical rules, not by its nonexistent "meaning". Also unlike modal auxiliaries, which do have meanings, be has none, even though it inflects for everything, even first person singular present tense (am) and first and third person past singular (was and were; no other verb distinguishes plural from singular in the past tense).

So if you want to say an auxiliary is not a verb, then be isn't a verb, though it has many verb-like characteristics. One of them is that it can be in the verb chain, that line of auxiliaries before you get to the main verb at the end. In fact, be has two places in the verb chain -- once to mark Passive

  • He was arrested.
  • He has been arrested.
  • He must have been arrested.

and once to mark Progressive

  • He was skating.
  • He has been skating.
  • He must have been skating.

which can occur together, though not often, because the be's bump into one another
(marked with ?? below)

  • We are being photographed.
  • ??We have been being photographed.
  • ??We must have been being photographed.

Note that the longest verb chains start with a modal auxiliary like must; that's because modals are the first auxiliary when they appear. That's the result of their being defective verbs without other forms. Be, have, and other auxiliaries require infinitives or participles to follow them; since modals don't have such non-finite forms, they can precede other auxiliaries, but they can't follow them.

If you were to ask my opinion, I'd say Auxiliaries are a variety of Verb. Why not?

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    As in If you don't be careful ... Commented Oct 5, 2018 at 21:11
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    Be is the auxiliary verb for predicate adjectives like careful and predicate nouns like a doctor. It can occur as an imperative (i.e, infinitive form) when the predicate word is active, like honest. But it's still an auxiliary. Commented Nov 7, 2019 at 17:58
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    Hi JL, but then why the use of DO above, when there would be no "support" needed? And when DO is in complementary distribution with other Aux? Commented Nov 7, 2019 at 19:42
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    Oops, had written +1, but it wasn't showing .. Commented Nov 7, 2019 at 19:46
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    Well, that's an argument for its non-auxiliarity; but then you have the problem of this one little exception in active imperatives. Is that a fact about be? Well, yes, but there are so many other strange facts about be that it gets lost in the crowd of exceptions. I'd rather say that it's a fact about imperatives with non-verbal predicates. It's the same fact either way, of course, but you can account for it in a number of ways. Commented Nov 7, 2019 at 22:32

If you are building a parser, you will separate modals, auxiliaries, and lexical verbs; mixing them up would be ungrammatical. You can lump them together as verbals (components of predicators), and equate that group with the historical verbs (named as a part-of-speech back when it was important not to over-complicate the analysis).

P.S. Beware the sentence fragment. Having a modal without another verbal does not force it to be a verb.

P.P.S. Modals could be stored like adverbs, or they could be stored as part of the auxiliary (components of memory are typically elided in discourse if they can be assumed); but 'adverb' is already an overloaded part-of-speech.


There are different views on the things you ask about and the major grammar texts do seem to differ in the details, so there are "traditional grammars" and "modern grammars" as authors grapple with various complexities and redefinitions and have different views.

I'm not sure about what you call a "proper verb" (a judgement call really) but the term 'main verb' is similar to what I think you mean.

Most simply:

A "simple verb" consists of a single "verb" (a 'main verb') in the "verb phrase".

A "compound verb" consists of more than one verb in the verb phrase.

These extra verbs are "auxiliary verbs", this later group consisting of the "primary auxilaries" ('have', 'be', 'do') and the "modal auxilaries" (a closed group of 9 including 'have', 'can', 'will'). [This is one classification system of verbs, though here not including copular verbs]

The auxilary verbs bring something extra to the verb phrase (for example assisting with indicating tense, mood, modality).

Modal auxilaries (like 'can') are defective in that they do not have all the properties of a 'main verb': they have no infinitive form, no regular adjustment for tense (excepting a historical use of shall/should for example), and only a single form for all persons. There are also the first verb in a verb phrase. They do not have past or present participle forms, so 'musting' is ungrammatical.

The primary auxilaries 'have' and 'be' can also function as main verbs. I think that 'can' can too but only when it not used as a modal auxilary, i.e. when it has a certain different meaning. One can provide the following advice to their teenage children playing music too loud in their room: "Can the noise now please!" (where 'can' means 'turn it off'), but this is like your note about canning factories - its a different meaning to say " 'to can' the beans" - from what you were asking about.

  • Thanks for "main verb". That sounds good. But is an auxiliary a verb? I've noticed a lot of people dropping "verb" from the phrase after it's been introduced. If modal auxiliaries are defective does that mean they're not verbs, they're just called verbs? Is "auxiliary verb" simply a phrase (that just happens to include the word "verb") that is used to describe a part of speech that is verb-like without being a verb? Must a verb include an infinitive form and a gerund as per my initial understanding? What is the minimum set of properties for any verb?
    – CJ Dennis
    Commented Dec 7, 2014 at 9:30
  • You're repeating the question that Kerry said has no definitive answer. It all depends on the particular grammatical theory you choose to follow.
    – Colin Fine
    Commented Dec 7, 2014 at 10:48
  • Wouldn't "Yes, we can!" or, "I surely must" be examples of auxiliary verb?
    – Oldbag
    Commented Dec 7, 2014 at 13:12
  • @ColinFine In that case can someone provide an example where in one grammatical thory "must" is a verb an in another it is not? Or are you saying that there is no consensus? That some people regard it as a verb as some others don't? Only Kerry's first paragraph was on point (albeit not clearly answering my question), the rest of the answer was explaining what the various auxiliary forms do.
    – CJ Dennis
    Commented Dec 8, 2014 at 1:48
  • @CJDennis: no I can't, because in my view auxiliaries are indeed a subset of verbs. I took it that you were suggesting such an analysis in quoting that view of French.
    – Colin Fine
    Commented Dec 8, 2014 at 23:13

I agree totally with you about the issue, except that modal auxiliary verbs should be classified as adjectives.

In "{Mary is} pretty"
  • is modifies Mary's existentialism, by being an adjective to Mary.
  • pretty modifies Mary's existentialism, by stating that {Mary's existentialism} being pretty.

However, the more established way to look at it is, in not quite precise explanation,

  • The actual verb is not is, but be.
  • is is merely an auxiliary form to the verb be.
  • Perhaps more precisely, is and being are inflections of be.
  • is is an auxiliary inflection to the word be.
  • The gerund/present-participle/verbal-noun of is is being.
  • The to-infinitive of is is to be.
"She can come"
  • can/could are auxiliary forms of the complex-verb/verb-phrase be able to.
  • The to-infinitive of can/could is to be able to.
  • The gerund/present-participle/verbal-noun of can/could is being able to.
  • Thus, "She is able", where is is the auxiliary inflection of be.
*She will sing"
  • will-1 is a complete verb of its own right.
  • to will is the to-infinitive. e.g. To will oneself to sing.
  • willing is the present-participle. e.g., she is willing to sing.

Similarly, have, need, want are complete verbs of their own right.

Some of the other modal auxiliaries and the actual inflectable verb-phrases they encapsulate:
  • will-2: be definite to.
    "She will die" = "She is definite to die".

  • may-1: will have possibility.
    "She may come" = "She will have possibility to come".
    "She might come" = "She would have possibility to come".

  • may-2: is allowed to.
    "You may come in" = "You are allowed to come in".

  • must-1: bear the imperative.
    "She must die" = "She bears the imperative to fail".

  • must-2: bear the obligation.
    "She must succeed" = "She bears the obligation to succeed".

  • must-3: carry a heavy presumed possibility.
    "She must have eaten my corn dog!" = "She carries a heavy presumed possibility of having eaten my corn dog!"

  • I wasn't suggesting that auxiliaries should be classified as adverbs, just that in my mind they behave in much the same way. "will" in "She will sing" is an auxiliary. What if she doesn't sing? "She would have sung". "will" changes to "would (have)" and "would" is not part of the verb "will". Consider "She will[aux.] will[v.] herself to sing".
    – CJ Dennis
    Commented Dec 8, 2014 at 1:58
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    You are mixing two different vectors of will: will-1 and will-2. Commented Dec 8, 2014 at 6:13
  • Are you talking about my last example "She will will herself to sing" or your example "She will sing"? In your example the verb is "sing" and "will" is an auxiliary. In my example I have used both and marked both with which one they are. "She will sing" means the same as "She is going to sing" which has a different meaning from "willing".
    – CJ Dennis
    Commented Dec 8, 2014 at 6:22
  • This is too confusing. Why do you bring up adjectives? What is an "auxiliary inflection"? On what basis do you say that the infinitive form of "can" is "to be able to"? Why not "to be allowed to"? (Can has both meanings, and arguably neither is more dominant than the other.) How do you distinguish must-1 and 2? Commented Dec 8, 2014 at 14:55

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