A construction involving a verb that marks it as referring to a future event. We tend to think of time as being past, present, or future, so might expect languages to use this three-way pattern. In fact, a simple division into past tense, present tense, and future tense is very rare: I can't think of any language that has it. Usually languages use overlapping categories of aspect (completed, incomplete, habitual etc.), intention, and observability. There are likely to be several kinds of past tense, while many languages lack a future tense altogether, just using the present.

English

English verbs are marked only for present and past. There are numerous ways to show the future. There is the auxiliary verb will and its contraction 'll; there is the triplet of be going to and a range of contractions from that; and there are both the simple and the continuous present tense forms. In older or more conservative registers there is another auxiliary shall, though I don't think there's any dialect that still consistently uses this.
(1a) I will visit Mary tomorrow.
(1b) I'll visit Mary tomorrow.
(2a) I am going to visit Mary tomorrow.
(2b) I'm going to visit Mary tomorrow.
(2c) I'm gonna visit Mary tomorrow.
(3) I visit Mary tomorrow.
(4a) I am visiting Mary tomorrow.
(4b) I'm visiting Mary tomorrow.
(5) I shall visit Mary tomorrow.

There's yet another possibility when we restrict the time to near future:

(6a) I am about to visit Mary.
(6b) I'm about to visit Mary.
Oversimplification: Probably the commonest in speech are (1b), (2c), and (4b), where gonna is an inexact rendition of various possible contractions. In writing the uncontracted forms are more often used, and gonna is avoided except in representing speech.

As for the difference in meaning between them, I leave that to you. There is no obvious distinction, except that (1a) would almost always be said with will emphasized, with a sense of intention or determination rather than future fact.

The use of will and shall as auxiliaries dates back into the Proto-Germanic, but not their use as simple future markers, which is a Middle English development. In Old English wille meant 'want to' and sceal meant 'be obliged to', and the present was used for plain future. These senses still exist when the auxiliaries are used forcefully or in law: I will go to the ball! You shall obey me! The goods shall be of marketable quality. From this derived the modern (though no longer current) English distinction of first person 'I shall' but second and third person 'you will, they will' for plain future.

Some other languages

In German the auxiliary used is werden, literally 'become', and wollen and sollen always have the forceful sense. The following verb, which goes to the end of the clause as all demoted verbs do in German, is in the infinitive or -en form: Ich werde Mary besuchen. In English only the verb be has a distinct infinitive form: I will be, I am going to be, not *I will am.

In Romance languages there are one-word future tense forms, whose endings derive from the infinitive followed by the inflected present of 'to have': French j'ai 'I have', chanter 'to sing', je chanterai 'I'll sing'; Italian ho 'I have', cantare 'to sing', cantarò 'I'll sing'. In Vulgar Latin this began as a compound locution cantare habeo (pronounced cantar aveo), replacing the Classical Latin future, which used the suffix -bi-: cantabis 'you will sing'. However, French (at least) has also developed a compound form with 'go': je vais chanter = 'I am going to sing'. (I am told Spanish and Portuguese also have, so this alternative might go back to Vulgar Latin too.) The grammaticalization of 'go' to 'future' is common.

In Classical Greek the future was also a single word, but was a new formation in Greek, not the same as the Latin. There was no common Proto-Indo-European future: each branch devised its own. Many futures were sigmatic, that is having a suffix -s-, as in grapho 'I write', grapso 'I'll write'. In later Greek a compound was formed with the verb thelo 'I want to', just like English will. In Modern Greek this has become the particle tha before the verb.

Finnish is one of those that have no distinct future. The present tense is used, with an adverb of time if necessary. However, gn0sis tells me Swedish-speakers sometimes imitate the Swedish construction and use 'to come' as an auxiliary.

Indonesian has no tense or aspect marking at all, but uses independent adverbs: saya minum 'I drink', saya akan minum 'I will drink', where akan is parallel to mau 'want to', sudah 'already', etc.

If I wasn't writing this at work I could scour my books looking for interesting systems in exotic languages, but as it is I'll just mention the Swahili: the prefix -ta- is used. From kuandika 'to write' we get nitaandika 'I'll write', parallel to ninaandika 'I'm writing', niliandika 'I wrote', and nimeandika 'I've written'. This comes from the full verb (ku)taka '(to) want'.

Some languages maintain a consistent distinction between so-called realis and irrealis forms. Realis are those that are real or can be verified: present tense or past tense known to the speaker. Irrealis forms include conditional and subjunctive, the equivalents of would or could. They may also mark hearsay reports as irrealis, and since there are strictly speaking no future facts, only suppositions or intentions, the future patterns as irrealis in these languages.

Log in or register to write something here or to contact authors.