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The English verb can is very strange for several reasons:

  • It drops the to on any infinitive verb forms that follow it. That is, unlike in the verb want in the sentence I want to eat, you would not include the infinitive to marker in the sentence I can eat.
  • Can itself has no infinitive form. A construction like I like to can understand foreign languages is not grammatical; you have to use the phrase to be able, as in, I like to be able to understand foreign languages.
  • Can does not conjugate. Most verbs add an s or es when they have a third person singular subject, but can does not.
  • You cannot add tense modifiers to can. I will can, I canned, I had canned and so on are all ungrammatical (unless you are talking about putting things in can). You have to use constructions like "I will be able to", "I could", "I had been able to", etc.

All of the other languages I know anything about do not share any of these characteristics in their corresponding verbs.

What are the historical reasons for why the verb can behaves so oddly in English?

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The verbs can, could, may, might, shall, should, will, would, must, all behave the same way. They're called modals, and other languages in the Germanic language family also have them. –  Peter Shor Jun 14 '12 at 15:53
    
Also, it's the only word I can think of whose negation is compressed into one word: cannot. –  JeffSahol Jun 14 '12 at 19:12
    
@JeffSahol: Notwithstanding fits too, I would say. And it’s not modern, but won’t used to be wynnot. Going back further, a lot of Old English words could be negated with n- (a shortening of ne), which is where we get never and none. –  Jon Purdy Jun 14 '12 at 21:46
    
...and on top of all that mess some idiot thought it's a good idea to give it a noun meaning too, describing a metal container... –  SF. Jun 15 '12 at 7:44

2 Answers 2

As Peter Shor indicates in his comment, these are modal verbs and are found in other Germanic languages as well. Their unusual conjugation is due to their historical source:

Germanic modal verbs are preterite-present verbs, which means that their present tense has the form of a vocalic preterite. This is the source of the vowel alternation between singular and plural in German and Dutch. Because of their preterite origins, modal verbs also lack the suffix (-s in modern English, -t in German and Dutch) that would normally mark the third person singular form.

Preterite-present verbs are a very old group of verbs found in Germanic languages that in turn descended from a set of Proto-Indo-European verbs. Originally these were perfect-aspect verbs that referred to a present state.

The common characteristic is that the "present tense" of these verbs was conjugated like the "past tense" (essentially). In Modern English this now means that the present-tense "-s" ending is not used, and that modifiers like "to", "will", etc. can't be added. It happens that these verbs are all modals in Modern English, which probably helps preserve the unusual conjugation.

So this is a bit of a historical accident that made a lot more sense in PIE, and now is frozen that way.

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I think the first-hand association would come from French; after all, 2/3 of the English words and grammar derive from French. (Which is not a Germanic language btw.) –  poepje Jun 14 '12 at 20:51
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@poepje many Modern English words did come from French, but our modal verbs all happen to derive from Germanic. Also, I don't know how you measure 2/3 of grammar. –  Mark Beadles Jun 14 '12 at 22:47
    
Well I suppose it's a somewhat 'loose' measurement.. But that's what I used to learn at high school anyway :) –  poepje Jun 14 '12 at 23:41

Modal auxiliary verbs in English are all defective (which means they are missing normal inflected forms, like infinitive, past tense, past participle, and 3rd Sing present), irregular, idiomatic, and very, very common. All the phenomena mentioned in the question are typical of modals so can is simply a normal modal in this regard.

Can does have a couple of unique peculiarities, though. All modals have at least a logical Epistemic sense, about possibility and probability:

  • This can't be the place.
  • This may be the place.
  • This must be the place.
  • He should be asleep by now.

and a social/personal Deontic sense, about permission and obligation:

  • You can't carry that on the plane.
  • You may leave your bag here.
  • She must leave immediately.
  • You should see a doctor about that.

and can is no exception.

However, Epistemic can occurs only in NPI environments, and is ungrammatical without a Negative trigger.

  • *This can be the place.

and can also has an additional sense, referring specifically to personal ability, rather than to permission or logical possibility:

  • She can run a marathon in under 2 hours.
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[Epistemic can cannot occur in environments other than NPI environments.] Yes it can! –  Edwin Ashworth Jun 16 '12 at 22:17
    
This can be true. Possibly. –  John Lawler Jun 16 '12 at 22:25

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