They are fixing an old chair. There is no context that this is some particular chair that the speaker is familiar with. In fact in Finnish there's no a and the words. When I learned English (as a person who is fluent in Finnish) we were told that the is some specific chair and we don't know that here and I was taught that in single sentences where there is no context about what chair we are talking about but it's just some chair it should be a chair.
All of these sentences require you to imagine them in context. That's the nature of translating individual sentence. Without doing this, you should have just as much of an issue with the use of "they" as with "the". Both rely on context to have meaning. If a complete stranger walked up to you on the street and said this sentence to you in English, you wouldn't only be wondering "The chair? Which chair?!!!!" but also "They? Who are you talking about!?"
I agree, without context one should not assume "the". The way I learnt it in the course, if the noun comes before the verb, it is "the", otherwise it is "an". Unless of course context is provided that says otherwise.
Tuolia (roughly "some of the/a chair") indicates that we're not talking about the completed (telic) action, but the incomplete (atelic) process before completion. This fits very well with the present continuous.
Also, we would just about never say "They repair an old chair" in English. It sounds habitual. "They repair old chairs" sounds fine, but if you habitually "repair" one single old chair, that's very strange. (With extra information in the sentence, it could mean different old chairs, for example: "They repair an old chair every Sunday" means it's a different chair each Sunday.)
¨Also, we would just about never say "They repair an old chair" in English.¨
I think that we could say that in a quite ordinary context:
IN A WOOD REPAIR WORKSHOP "Are they still working on the theater´s new chairs at the workshop? - No, They finished them. They start now to repair an old chair¨.
I'm also curious about this. From this: https://en.wikipedia.org/wiki/Finnish_noun_cases, it seems like it has to do with the action being finished or not. As they are fixing the chair, it would mean that they are not finished with their repairs yet. That is, there are some parts of the fixing left to do, and therefore the direct object is in the partitive case.
This is just my guess as to why this case is used. I hope some native could shed some light on it.
In this case you can say:
1) He korjaavat tuolia or 2) He korjaavat tuolin
- They are repairing a/the chair: They are currently doing so (unless there's some other word that indicates it will happen later), they won't get the job done or you don't know if they will or not, they may be repairing it partly.
- They repair a/the chair or they will repair it: They are doing it currently or in the future and they will get the whole thing completely done.
It's the partitive, i guess. At least that's what I can gather from the wikipedia page on finnish grammar: https://en.wikipedia.org/wiki/Finnish_grammar#Cases.
There are about 15 cases in Finnish, so it will get worse. I'm saying about 15 since the accusative is weird, and can either be a single case, several cases or not really a case, depending on how you analyse it. Fortunately it's quite systematic, so it's definitely possible to learn it. The latin case system is far worse, although it has only 7 cases.
I do however wish the course creators would give us a primer on the different cases as they appear. The partitive has now been used in three skills without being explained.
I think people are relying too much on Duolingo as the only way to learn Finnish. Everytime we don't understand something, it is very likely that we will find the aswer in the comments, now that the course has been going for a month. Using that information, there are plenty of resources in the internet to figure out how the language works gramatically. In fact, just the effort we'd put on doing some research would help us remember what we're learning much more than just repeating.
Duolingo is a good tool, but not the only one.
If you login on a desktop computer or notebook you will find grammar and other helpful explanations for every topic - also for the partitive. I don't know why the information is not provided in the app for Finnish (some languages do have this button next to the key for unlocking a stage...)
Before a consonant, the indefinite article is 'a'. Before a vowel, the indefinite article is 'an'.
Before a consonant, the definite article is 'the', typically pronounced /ðə/, rhyming with 'uh'. Before a consonant, the definite article is still 'the', but may be pronounced /ði/, sounding like archaic 'thee'.