Why can't the word "can" be used in future tense (will can)?

I'm curious about why the English word can cannot be used in future tense (e.g. will can).

An example unrelated to English is French term je pourrai, but that's exactly what I mean.

Compare German ich werde können which translates exactly to I will be able, and literally to I will can, given that können and can have the same origin. I feel that this is confusing.

Answer

This is a good example of the problems caused by lying to students in saying that will is "The Future Tense". There is no future tense in English. There is likewise no perfect tense, no progressive tense, no pluperfect tense, no future perfect tense. There are also no moods or voices. No matter what you've been taught. Sorry about that.

What English has is a present tense and a past tense, both managed by suffix or root change (deletes/deleted, drives/drove). That's it for tenses. There are various constructions like the Perfect construction, the Progressive construction, the Passive construction, etc. All of them take several words and don't require endings or prefixes, and word order is important.

One of the constructions that occurs in practically every sentence is the Verb Phrase, which usually starts with an auxiliary verb of some sort: some form of be or have for Perfect, Passive, or Progressive, and, at the beginning of the verb phrase, a modal auxiliary verb. When modals occur, they are always at the beginning of the verb phrase, because they only have one form (they are "defective verbs"), and that form is not an infinitive form or a participle form, so it can't go after be or have as the constructions require.

This results in modal auxiliaries always occurring alone at the beginning of a verb phrase -- or inverted with the subject in questions -- whenever they occur; and it also has the effect of limiting modals to one per verb phrase, at the beginning.

Why is this relevant to the "future tense"? Because what students are erroneously taught is not that will is one of the modal auxiliaries, and therefore behaves like can, may, must, should, would, could in not appearing together, but rather that will is "The Future Tense", a different category entirely, which can apply to anything, including uninflectable modal auxiliaries like can. Hence the question.

The answer is that English has special constructions that mean the same as modals, but have infinitive and participle forms, so they can be used in past tense, or in the Perfect or Progressive. These are called Periphrastic Modals, and the one associated with can in the sense you indicate is be able to.

That is, one can't say

• *I will/should can do that by next year.

but one can say, with the same intended meaning,

• I will/should be able to do that by next year.