Dear fellow duolingians, especially those of you who are committed to learn swedish to a level of near perfection. The time is ripe to take on the ambiguity of SKA.
To my learning ear, the sentence "Han ska sluta dricka kaffe." sounds like "he shall stop drinking coffee" in it has some sort of normative "ring", he shall stop drinking coffee ( and maybe repent ) :-) .
Jokes a part I suspect that regular swedes in everyday circumstances would mostly use the present tense to convey the meaning of an action happening in the near future unless there is any specific need to use an alternative form.
If the premise is correct the most common translation should look like
(1) han slutar dricka kaffe ... ( i morgon .. nästa vecka ... börjar på måndag.. o.s.v )
The second most common form... would be using the kommer att
(2) Han kommer att sluta dricka kaffe
And only in the third place... maybe the event is very far away in the future.. maybe we want to express a strong resolution about the all coffe thing..
(3) "Han ska sluta dricka kaffe."
It sounds also rather formal, kinda solemn.. in a way...maybe too formal for such a thing as stopping to drink coffee. (maybe)
But these are merely my thoughts and impressions as a neophyte.
I'm not entirely sure I'm interpreting you correctly, but I'm afraid I think you may be a bit off. Using ska here is very versatile - I might say nä, nu ska jag sluta dricka kaffe which would be the immediate future; and I might say jag ska sluta dricka kaffe nästa år and mean that I'm going to quit coffee next year.
It should be accepted when translating, but since you're writing here I guess you had it as a dictation exercise. Unfortunately, for those we cannot add any spelling variation whatsoever.
PS it's true that skall is more common in the written language than it is in the spoken language (where it is virtually not used) but ska is more common than skall in the written language, and nowadays it is recommended to use ska even in formal texts.
The sentence is ambiguous: it could mean, as you have translated, that he has decided to quit drinking coffee but could also refer to the fact he should quit because it is not good for him,in other words an advise from a friend,wife or doctor. Is there a way to make sure that everybody understand that he has decided to quit? Frankly the only cue is that this is lesson about future sentences.
Tack så mycket. However, I believe it is a bit more subtle. For example, Babbel, the dictionary on line, offers one definition for 'sluta' as 'to conclude' and gives the following verbs as synonyms: avsluta, ingå, dra slutsatsen, slutföra, avgöra, komma fram till, göra upp, konkludera, dra en slutsats. It also provides the following example: "Europeiska unionen vill sluta ett frihandelsavtal med Colombia" = The European Union is planning to conclude a free trade agreement with Colombia. Of course, 'conclude' in this sentence is synonymous with 'finish'. I appreciate your help.