Vrftjkry

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.

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.

Per Wikipedia, can is a "defective verb"...

For example, can lacks an infinitive, future tense, participle, imperative, and gerund. The missing parts of speech are instead supplied by using the appropriate forms of to be plus able to. So, while I could write and I was able to write have the same meaning, I could has two meanings depending on use, which are I was able to or I would be able to. One cannot say I will can, which is instead expressed as I will be able to.

As you'll see from that Wikipedia article, many other languages (including French and German as mentioned by OP) have defective verbs. But there's no particular reason why they should be the same verbs in different languages, since the reason for their existence at all (natural language variation over time) will depend very much on individual circumstances relating to time, place, meaning, and peculiar factors relating to such things as the social class of different speakers, etc.

The reason is that standard English prohibits the use of double modals, which 'can' and 'will' both are, as addressed in this post. Some dialects, like my own, incorporate double modals like 'might could,' 'might should,' 'ought to should,' etc. but I don't believe I've ever heard 'will can.'

Can is part of the set of verbs called the 'Preterite-present' (https://en.wikipedia.org/wiki/Germanic_verb#Preterite-presents). This essentially means that the form can was originally a past tense form which has now taken on a present tense meaning. It betrays its past tense origin in English because it lacks the third person singular s (compare he holds vs he held); other languages offer even more proof (e.g. the vowel shift between Dutch singular kan and plural kunnen).

For reasons unknown to me, English modals did not 'develop' new infintival forms. This did happen in Dutch, German, Swedish, ... which is why he will can is perfectly possible in those languages (hij zal kunnen/er wird können/han ska kunna). Can as it is now still 'functions' as a past tense form and much like how you can't say he will held, you can't say he will can. As others have pointed out, this makes the verb defective.