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?
Best Answer
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:
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.