Learn Hardcore Finnish: Päätän tuoda paperit huomenna, jotta saan leiman silloin. - I decide to bring the papers tomorrow so that I can get the stamp then.

Finnish commonly uses the present tense for future plans when a time word makes the future clear. Here huomenna (tomorrow) signals the future, so päätän can mean I decide / I’m deciding (now) that I will….
If you wanted to emphasize that the decision was made earlier, you’d use past: Päätin tuoda paperit huomenna… (I decided to bring…).

Why does Finnish use päätän + tuoda (an infinitive) instead of a form like tuon?

After verbs like päättää (to decide), Finnish typically uses the A-infinitive (the dictionary form): päättää tehdä/jättää/tuoda… = decide to do/leave/bring….
So päätän tuoda literally matches I decide to bring. Using tuon would make it a separate finite clause and would require a different structure.

What exactly is the form tuoda here?

Tuoda is the 1st infinitive (A-infinitive) of the verb meaning to bring. It’s the form used after many verbs expressing intention/decision/ability, e.g. haluan tuoda (I want to bring), voin tuoda (I can bring), päätän tuoda (I decide to bring).

Why is it paperit and not something like paperia?

Because this sentence treats the papers/documents as a complete, definite set you will bring (a total object).

paperit = total object, the papers / the documents (all of them)
paperia (partitive) would suggest an indefinite amount or not all of it, like some paper or some of the paperwork, depending on context.

Is paperit actually accusative here? It looks like nominative plural.

Yes: for many nouns, the accusative plural is identical in form to the nominative plural. So paperit can function as a total object (accusative plural) even though it “looks” nominative.
You can see the difference more clearly in singular objects like leiman, which has the visible -n accusative ending.

Why is it leiman with -n?

Leiman is the total object (accusative singular) of saan (I get). The -n marks that it’s a complete, countable result: you get one stamp (or the stamp) as an outcome.
If you wanted “some stamp/partial/indefinite” (rare here), you might see a partitive form instead, but with saada + a concrete item, total object is the default.

What does jotta do here, and how is it different from että?

jotta introduces a purpose/result clause: so that / in order that. It answers “what for?”

…, jotta saan leiman silloin. = …so that I get the stamp then.
että more often introduces a content clause (“that…” as information), not purpose:
Tiedän, että saan leiman. = I know that I will get the stamp.

Should the verb after jotta be conditional, like saisin?

Often, yes—Finnish frequently uses the conditional in purpose clauses:

…, jotta saisin leiman silloin. = …so that I would get the stamp then (purpose/goal phrasing; very common) But the present is also possible:
…, jotta saan leiman silloin. = …so that I get the stamp then Using the present can sound a bit more direct and “matter-of-fact” (as if the result is expected), while the conditional often sounds more neutral as a goal.

Why is there a comma before jotta?

Because jotta starts a subordinate clause, and Finnish normally separates subordinate clauses with a comma:

Päätän tuoda paperit huomenna, jotta…

What does silloin refer to, and could it be replaced with sitten?

silloin means then / at that time, pointing to a specific time already mentioned or understood—here it refers back to huomenna.
sitten is more like then / after that / next in a sequence of events. In this sentence, silloin fits better because it ties the stamp to the time tomorrow.

Why is the subject minä not written?

Finnish usually drops personal pronouns when the verb ending already shows the person.
päätän and saan clearly indicate I, so minä is optional and would mainly add emphasis or contrast:

Minä päätän… = I (as for me) decide…

Could the word order be changed?

Yes, Finnish word order is flexible and often changes for emphasis:

Päätän tuoda paperit huomenna… (neutral)
Huomenna päätän tuoda paperit… (emphasizes tomorrow)
Paperit päätän tuoda huomenna… (emphasizes the papers/documents, maybe contrasting with something else)

Is tuoda definitely “bring”? If I’m going to an office, wouldn’t it be “take”?

Finnish distinguishes tuoda (bring) vs viedä (take) based on viewpoint.

tuoda = bring toward the relevant place/person (often the listener or the place being talked about)
viedä = take away/from here to somewhere else
If you’re speaking from the office’s perspective (or to someone at the office), tuoda paperit is natural. If you’re speaking from home about going elsewhere, viedä paperit can also be natural. Both can be correct depending on viewpoint.