Is it natural that "future possibility" is a separate tense?

Question

I had some thought on how tenses work in my conlang. Eventually, I built a model and defined the tenses accordingly. But before showing that model, let be introduce the most basic tense in my conlang:

1. Tautological tense. This "tense" states that the sentence is an absolute truth independent to time.

• Example English sentence: "One plus one equals two."

The usual partition of time is Past, Present, and Future. But there is a problem. Since "present" is a single timestamp when taken literally, almost no statement will hold "at the present" and no other time.

So my model makes partition of time differently. It's History, Progress, and Prospects. That gives some more tenses:

2. Simple past tense. This tense states that the action in statement has happened before.

• Example English sentence: "I went to swimming."

3. Present progressive tense. This tense states that the action in the statement is in progression.

• Example English sentence: "I am going to swimming."

But unlike the history or the progress, there is a distinctive feature about the prospects. There is no guarantee how the future will become reality. In other words, I should treat the future as a multiple world, hence the plural "prospects". (You can compare this model to the many-worlds interpretation of Quantum Mechanics.)

This gives two future tenses:

4. Universal future tense. This tense states that the action in the statement will happen anyway.

• Example English sentence: "I will go to swimming."

5. Existential future tense. This tense states that the action in the statement might happen in the future.

• Example English sentence: "I might go to swimming."

But I wonder it's natural to give such distinction in the future tense. Is it?

Sidenote: There isn't going to be a simple present tense in my conlang. My conlang cannot just state "I go to swimming." However, my conlang can state "I go to swimming everyday." because the adverb "everyday" takes the quotient of time by each day. That enables the present progressive tense usable.

Answer 1

Instead of "tautological", the usual name for that in linguistics is gnomic aspect. Lingála, for example, has this type of marking.

Similarly, your "historical" marking is usually called perfective aspect, indicating that something is over and done with. The opposite, the "progressive", is also called imperfective aspect, from the Latin for "unfinished". Many languages have a basic distinction between perfective and imperfective rather than past and present (as in English): you find this in many Slavic languages, and also many Semitic languages.

As for your future, this is not uncommon: the marking used for hypotheticals and possibilities is usually called the subjunctive mood. This one is also found in Lingála, as well as Latin and various others. This name comes from the fact that, in Latin, it's often used in subordinate clauses; an alternate name that better describes its meaning is irrealis mood, from the Latin for "not a fact" (the opposite is realis, "a fact").

Answer 2

To address the "future possibility": Spanish subjunctive mood is used to describe possibilities, attitudes or wishes. Historically, future subjunctive was very much alive, but has almost completely disappeared from contemporary language (and the subjunctive itself, past or present, is slowly being eroded away as well.)