I love this discussion! It makes me realize: some things are easy, but not always simple. I think I will just accept the duolingo way, and hopefully when in Finland experience how people use it. That's just practical, I guess. But I really do like these discussions as they give flavour to an otherwise black and white set of rules. Thank you all!
Both articles are possible. The partitive form suggests ongoing, unfinished action. The object can be a chair that's been mentioned previously ("the") or a chair that has never been mentioned before ("a"). The accusative form tuolin would mean that the action has been finished. Again both articles are possible. This is something called aspect and in English aspect is mainly a verb feature, although in Finnish it's not.
- Me maalaamme tuolia. We are painting a/the chair. or We will be painting a/the chair.
- Me maalaamme tuolin. We will paint a/the chair.
- Me maalaamme tuoleja. We paint chairs.
tuoleja is the partitive plural and that's the go-to object form if you want to use the English verb "paint" without the ing. Although there is a theoretical chance, in which Me maalaamme tuolia can be used in the meaning "We paint the chair". (Like answering the question, "what do you do every Tuesday between 3pm and 4pm?" and you paint the same chair every single Tuesday.) :)
"Tuolin" just specifies that you're indeed going to do it completely, but "Me maalaamme tuolia." doesn't count the possibility out that you're also going to finish the painting.
"Me maalaamme tuolia." is always an ongoing progress, but it could be said for example at home, when you're talking about what you're doing at your workplace (while the painting is not yet done).
"We paint the chair" sounds either like some kind of repetitive action, which sounds like a bit of a stretch if it's just one chair (still possible though), or like a description of what happens in a book or a movie, something that's frozen for all time, and in that case, "we paint the chair" implies that it is a completed action, in which case it would be Maalaamme tuolin. The atelic aspect indicated here in Finnish is best translated with the present (or future) continuous in English, indicating the action underway, not as a completed action.