Anderson's Essentials of Linguistics says that in English:

The modal auxiliaries never change their form: they occupy the T- head position in their own right.

The non-modal auxiliaries, like main verbs, change their form depending on what tense feature is in the T-head position, among other things.

bare (non-tensed) be have do
[-past] (tensed) am/are/is has does
[+past] (tensed) was/were had did
past participle (non-tensed) been had done
present participle (non-tensed) being having doing

Are "can, could" different forms of the same modal auxiliary?

Are "may, might" also?

3 Answers 3


Are "can, could" different forms of the same modal auxiliary?

Are "may, might" also?

Historically yes, but they had some semantic differentiation.

The same for pairs shall : should, will : would.

There are also archaic forms of second person singular with suffix -(s)t, which would be used with pronoun thou:

I can : thou canst

I could : thou could(e)st

I may : thou mayst

I might : thou might(e)st

I mote : thou *must (← Middle English most ← Old English mōst)

I must : thou mustest

(must is Past Tense of mote ← Middle English moten ← Old English mōtan)

I shall : thou shalt

I should : thou should(e)st

I will : thou wilt

I would : thou would(e)st

  • I upvoted, but I'm not sure there has been a significant semantic shift. The simple past stem is also used for the subjunctive mood, hence the "if it were" construction, and most instances of could, should, would and might are as subjunctives of their present tense counterparts - especially will, the future tense indicator.
    – No Name
    Commented Jul 10, 2023 at 0:28
  • 1
    Perhaps worth noting that must was the past tense of the now largely unused verb mote (still used in a set phrase in freemasonry)
    – Henry
    Commented Jul 10, 2023 at 0:42
  • @Henry: I added mote, with my guess of thou *must form for this verb (I do not know if it is attested in Modern English).
    – Arfrever
    Commented Jul 10, 2023 at 2:23
  • Elizabeth Barrett Browning started Sonnets from the Portuguese 14 with "If thou must love me"
    – Henry
    Commented Jul 10, 2023 at 6:44
  • That poem does not shy away from using art, wast, canst, shalt, wilt, would'st, may'st, but it does not contain any mote, so I feel that it rather treats must as inconjugatable for person...
    – Arfrever
    Commented Jul 10, 2023 at 7:22

Yes, modal auxiliary verbs do not inflect, hence they never change their form.

  • may, shall, will, and can are formed from present stems
  • might, should, would, could, and must are formed from past stems
    (there used to be a present-stem like must back when modals inflected)
  • need and dare can be modal inside a negative field (i.e, semimodals are Negative Polarity Items)
    _ Need you ask? You need not/needn't worry; He dare not/dassn't speak her name._

Semimodals are optional; one need not treat need or dare as modal. They're always grammatical when inflected, but they need a to before their infinitive complements.

  • I don't need to ask him; I need to tell him.
  • I need not ask him; I need to tell him.

Since modals don't inflect, they have no tense; and since they must be followed by an infinitive verb form, they're limited to the first slot of the verb chain, which means any sentence with a modal in the matrix verb phrase renders the sentence tenseless.

  • 4
    But this description doesn't account for the fact that when back-shifted, can and will are replaced by could and would. (There are some other examples, but they're less common).
    – Colin Fine
    Commented Jul 9, 2023 at 17:44
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    It also doesn’t account for the fact that, if you insist that modals do not inflect, that can/could, etc., are separate modals, and that dare without to is a modal, you implicitly also create an extra modal dared, which has no purpose whatsoever than to supply a modal equivalent to dare for use in past-tense contexts like, “I daredn’t look”. And of course the messiness of modals being uniquely conditioned on utterance time to begin with (i.e., “*I can’t go yesterday, but I could tomorrow” is ungrammatical). It also provides no evidence of its claim. So -1 from me, I’m afraid. Commented Jul 9, 2023 at 17:57
  • 1
    "since they must be followed by an infinitive verb form, they're limited to the first slot of the verb chain" <-- The latter fact doesn't follow from the first there, does it? [It's not quite the case that all modals are followed by infinitives. For example, would appears in the PPI idiom would rather, which is often followed by a finite clause.] Commented Jul 9, 2023 at 18:56
  • @JanusBahsJacquet There’s some question about how daredn’t really worked. In any event, contracting that is no longer possible in present-day American English, if it ever was so (citation needed). And of course it will also have competed with dursn’t. Check out the mentions of neededn’t and usedn’t at that same link: they might simply never have happened in speech.
    – tchrist
    Commented Jul 10, 2023 at 1:05
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    Need and dare are in transition, and like any modal, they have lots of odd expressions and constructions. Not everything can be explained by labelling. BTW, I misspoke above; the reason modals are restricted to first position is that all the rest of the possible verbs in the chain (do, be, have, get, etc.) must appear either tensed (which modals can't) or in an infinitive or participial form, which modals also can't since they have only one form.
    – jlawler
    Commented Jul 10, 2023 at 17:16

In English, what makes a modal (morphologically) a modal is three properties: use as an auxiliary, the absence of either participle, and the simple present stem inflecting as if it were the simple past stem - that is, the third person present singular uses the bare form rather than a hypothetical -s form. With the loss of "thou" and the slow but steady death of the subjunctive, it certainly looks like the modals don't inflect at all, especially when grade schoolers are taught all nine modal stems as separate words, rather than the four and a half they actually are. And this isn't even getting into the semi-modals "need" and "dare" that @jlawler talks about.

What does the subjunctive have to do with modals? Well, the subjunctive uses the simple past stem, hence "if it were". When "will" became the future tense indicator, it's past stem became the future tense indicator of counterfactual conditionals, where the subjunctive is used.

If I give them a present, they will like me more.
If I gave them a present, they would like me more.
If I had given them a present, they would have liked me more.

See the difference? Similarly for "can/could":

If I study, I can pass.
If I studied, I could pass.
If I had studied, I could have passed.

Note how the modal absorbs the subjunctivity of the main verb - it's "could pass", not "could passed" or "can passed", and similarly with "could have".

"May/might" and "shall/should" behave the same way:

If you kiss me, I may kiss back.
If you kissed me, I might kiss back.
If you had kissed me, I might have kissed back.

(This is only an example, not an invitation.)

If the user breaks the widget, he shall fix it.
If the user broke the widget, he should fix it.
If the user had broken the widget, he should have fixed it.

The fact that "should" carries less weight than "shall" is a side effect of suppletion by "must" - which does carry the full weight.

If the user broke the widget, he must fix it.

Speaking of, "must" is the only modal about which Anderson is completely correct - the present stem has been wholly replaced by the past stem in all cases, so it truly never inflects.

And this on top of the fact that "could" still sees use as "was able to", and "should" as "was required to" - the legitimate past tenses of their respective present stems.

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