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?