To answer the official question, in every sentence (not every clause, but every sentence) the first verb in the main verb phrase must be one of
- a Present tense form (am, is, are, have, has, does, do, or
Verb
+ -Z₃
, the 3SgPr inflection)
- a Past tense form (was, were, had, did, or
Verb
+ -ED
, the Past inflection)
- a Modal auxiliary verb (can, could, may, might, will, would, shall, should, must)
English modal auxiliaries are not inflected for tense, so they are either not in any tense or they are always present tense, depending on what kind of theory of tense you're applying.
So either all English sentences are in a tense, or there are some that aren't. But that's just how one describes the language -- it doesn't affect the grammar.
Time in English is frequently indicated by tense, but often enough it's also -- or even only -- a matter of the words or constructions used. See the Deixis Lectures for more on expressions of Time.
English tensed verb forms, however, often specifically don't refer to time. For instance, the Present tense, when used with an active verb, is most likely to refer to an habitual occurrence than to the present time. E.g,
- Bill walks to school means he walks (almost) every time he goes to school.
- That dog bites means that the dog has been known to bite people on some occasion(s).
- Mary drives a Toyota means that Mary usually drives (and probably owns) a Toyota.
None of these refer to what Bill, the dog, or Mary are doing at the present time -- neither the time of speaking nor the "present" of a narrative. This is called a generic construction.
The particular use in the original question licenses the use of a past verb form to indicate an unreal supposition, much the way certain regular subjunctive verb endings do in European languages; but only sporadically -- not regularly. This counterfactual conditional construction, like most archaic remnants, is idiomatic, and governed by only a few constructions and verbs. So one finds
- I wish I were home now.
- If I were you, ...
- If I had the money now, I'd give it to you.
English has no future in the future because English has no future tense at all. English verbs approach tense from two perspectives: before now (past), now and after now (present/nonpast). As such, we can conjugate the verb to eat as follows:
I eat.
I ate.
But there is no way to conjugate the verb for the future, and so we resort to periphrastic constructions to form future aspects, which, for better or for worse, usually infuse other meaning into the sentence:
I will eat (volition).
I shall eat (obligation).
I may eat (possibility/permission).
These all imply future time (and thus form the future aspect), but may infuse undesired meaning into the sentence. Nevertheless, we also have less meaning-rich, albeit more verbose, ways of expressing future time:
I am going to eat.
I am about to eat.
So, although there is no future in the future tense, we can form a future in the future aspect by combining the foregoing constructions:
I will be going to eat.
I will be about to eat.
Both of which sound fine on occasion, but may grate on the ears (eyes) if heard (read) too often, especially in the passive voice: the food will be going to be eaten.
It is also worth noting that the present tense is often used for both present and future time, often making the future aspect seem too verbose where it is still grammatical. Consider the following pairs:
I am going home tomorrow. / I will be going home tomorrow.
He heads out in an hour. / He will head out in an hour.
In each pair, both sentences mean about the same thing and, at least where I live, the average Joe is more likely to say the first. This is merely something to consider, however, and it is not meant to discourage your idea at all.
Best Answer
He will resign two months later is perfectly grammatical and idiomatic, but means "later than the (future or possible) events I am talking about". It would not be understood as "two months from now".
So The deal will be completed next May and he will resign two months later unambiguously says that he will resign in July.
Conversely "in two months" means "two months from now". The deal will be completed next May and he will resign in two months I would find confusing: I'd probably conclude that it meant he would resign in July, but I would not be sure.