"They are fixing the old chair."
Translation:He korjaavat vanhaa tuolia.
Because the action is atelic, as in possibly incomplete. If the action were telic, for example "They fixed the old chair", the Finnish translation would use accusative case, as in "he korjasivat vanhan tuolin". Both partitive and accusative can be used about the object of a future action, but it changes the meaning. "He aikovat korjata vanhan tuolin" means "They intend to fix the old chair", while "He aikovat korjata vanhaa tuolia" means "They intend to be fixing the old chair (but not necessarily until it's completely fixed)".
Don't worry! While "partitiivi" is one of the favourite swearwords of my students even after they have reached a very advanced level of Finnish, making mistakes with it is not the end of the world. Duolingo can be very harsh with mistakes like these that are easy to make but not always that relevant to being understood (like picking a wrong article in German). Do start worrying about it when you're writing work emails or residency applications in Finnish.
(I'm not saying it doesn't matter, just that it doesn't signal "hello, I'm a complete beginner with Finnish" the way an unconjugated main verb does, for example.)
It means that the action is incomplete. In English this is expressed by using the verb rather than the object, which is why the verb "to fix" is in the continuous form, "are fixing". And like Kristian says, this sentence can also refer to future action: "They will be fixing the/a old chair." :)
It's because 3rd person verbs are ambigious about what the subject could be, so the subject needs to be there if its in 3rd person. A singular 3rd person verb could have "se", "hän", and any singular noun as its subject. A plural 3rd person verb could have "ne", "he", and any plural noun as its subject. 1st and 2nd person verbs can only have one particular pronoun as their subjects, so the presence of a 1st or 2nd person pronoun is not necessary if there is a 1st or 2nd person verb.
Ah, thereby hangs a tale. Partitive case is applied to...
- ...noun phrases modified by a singular numeral (this does not apply to “yksi”, which is technically a pronoun, not a numeral). “Kaksi poroa” = “Two reindeer”
- ...noun phrases modified by an expression of an amount. “Suomessa on monta järveä” = “There are many lakes in Finland”
- ...plural noun phrases with indefinite referents in existential clauses and ownership clauses even when not modified by a numeral or an expression of an amount, i.e. the number/amount is unspecified. In other words, it is used in words that would usually be modified by “some” or “any” in an English translation. If the referents are definite, nominative case is used instead. “Kaupassa on karjalanpiirakoita” = “There are (some) Karelian pasties in the shop”
- ...mass nouns (usually). “Pullossa on viinaa” = “There is liquor in the bottle”
- ...the target of an ongoing action. “Rakennamme saunaa” = “We are building a sauna”
- ...the target of an irresultative action. “Halasin karhua” = “I hugged a bear”
- ...negated noun phrases in an existential clause or an ownership clause. “Minulla ei ole autoa” = “I don’t have a car”
- ...the subject of an existential clause or an ownership clause when the clause is in the form of a question and the inquirer either isn’t sure what answer to expect or expects a negative answer. When the inquirer is sure that the answer will be positive, nominative case is used instead. “Onko teillä juustohöylää?” = “Do you have a cheese slicer?”
- ...noun phrases preceded by most prepositions. “Ilman pipoa” = “Without a woolly hat”
- ...probably some other not-so-common situations that I can’t think of right now.
The algorithm can be a bit dumb sometimes. When something goes wrong it recommends the closest possible translation. My guess is that you used the verb form korjaa rather than korjaavat in your answer and got recommended an alternative translation rather than the primary translation which you can see at the top of this page. ne korjaa is informal spoken Finnish; he korjaavat is Standard Finnish. :)