I won't have finished my work until Saturday.

[beachboy]

1 - By Saturday I will have finished my work.
2 - I won't have finished my work until Saturday.

How common is it, in everyday English, to use the Future Perfect? Would there be more natural ways to express the same ideas as in the sentences above (which I myself came up with) with no or little change in meaning? What would a native say?

[Piscean]

How common is it, in everyday English, to use the Future Perfect?

It's reasonably common and natural.

[abaka]

Every one of the English verbal tenses (18 with the emphatic forms) exists because it is reasonably common and natural.

Precision in specifying temporal relations is absolutely fundamental to the way the native speakers of the language learn to think.

[Piscean]

Every one of the English verbal tenses (18 with the emphatic forms)

I'll resist the temptation to challenge that number - eighteen.

[abaka]

I'll resist the temptation to challenge that number - eighteen.

I'm too weak to resist. In the indicative only, uncoloured by modality, conditionality, or subjunctives:

Present: I run -- I am running -- I have run -- I have been running -- I do run
Past: I ran -- I was running -- I had run -- I had been running -- I did run
Future: I'll run -- I'll be running -- I'll have run -- I'll have been running
Future reported in the past: I'd run -- I'd be running -- I'd have run -- I'd have been running

[Piscean]

Present: I run -- I am running -- I have run -- I have been running -- I do run
Past: I ran -- I was running -- I had run -- I had been running -- I did run

I'd say we have two tenses there, unmarked (the traditional present) and marked (the traditional past). You have shown for each three/four forms/aspects/whatever in addition to the simple - the progressive/continuous/durative, the perfect/retrospective, a combination of the last two, and the emphatic.

Future: I'll run -- I'll be running -- I'll have run -- I'll have been running
Future reported in the past: I'd run -- I'd be running -- I'd have run -- I'd have been running

You did say that you were going to present forms 'uncoloured by modality'. Is will not a modal?

[abaka]

We're about to be exiled to Language Discussion, I think.

(1) English verb tenses as taught both to learners and to schoolchildren combine linguistic tense and aspect, thus producing 4x4+2 = 18 tenses, or perhaps 16 if the emphatic do forms are to be considered modal.

(2) Will can definitely be modal, to mark desire as opposed to the obligation carried by shall. Unlike shall in contemporary speech however, it and its past would are also, and more importantly, neutral markers of future and reported-future time. In

I will write my final exam tomorrow
He told me last year that by this summer he would have been studying English for half his life

the auxiliaries will and would carry no more modality than the auxiliaries have or been.

As for phrasings such as he used to go there every day and he is going to tell her tomorrow, I believe they are auxiliary in English in the same way as the periphrastic forms are auxiliary in in Latin. (Obviously the only strictly non-periphrastic forms in English are the simple present and past.)

[Piscean]

We're about to be exiled to Language Discussion, I think.

Quite probably.

(1) English verb tenses as taught both to learners and to schoolchildren combine linguistic tense and aspect, thus producing 4x4+2 = 18 tenses,
or perhaps 16 if the emphatic do forms are to be considered modal.

The number of forms presented to learners varies considerably:
Some teachers/writers present only the present and past (simple) forms as tenses;
Many include progressive/continuous and perfect forms (present/past progressive, perfect and perfect-progressive);
Some still include forms with will as future tenses (future progressive, perfect and perfect-progressive);
I have even seen forms with would presented as tenses (conditional and/or future-in the past);

I have never before seen forms with emphatic do presented as tenses or modal.

(2) Will can definitely be modal, to mark desire as opposed to the obligation carried by shall. Unlike shall in contemporary speech however, it and its past would are also, and more importantly, neutral markers of future and reported-future time.

I don't agree. The use of Will/would assert certainty, contrasted with lesser degrees of certainty implied by other modals. Will/would are no more or less 'neutral' than BE going to. Interestingly the certainty of will can be about present as well as future situations - He will be there now vs He will be there tomorrow. BE going to is used only of future situations.

As for phrasings such as he used to go there every day and he is going to tell her tomorrow, I believe they are auxiliary in English in the same way as the periphrastic forms are auxiliary in in Latin. (Obviously the only strictly non-periphrastic forms in English are the simple present and past.)

I consider those two forms to be aspects, habitual (used to) and prospective (BE going to), but accept that not many use these terms.

[Rover_KE]

We're about to be exiled to Language Discussion, I think.

This thread could run and run and has been relocated. I don't know why anybody should use exiled, banished or excommunicated when the OP has been asked and answered and the subsequent debate goes well over the heads of our client ESL students.

[Tdol]

We're about to be exiled to Language Discussion, I think.

(1) English verb tenses as taught both to learners and to schoolchildren combine linguistic tense and aspect,

That's the issue- I agree with Piscean about there being two tenses, but accept that most learners will have been taught differently. I teach future forms rather than a tense, but don't worry when people call it a tense.