That would be Miksi hän korjaa verannan?
Repair in the simple present tense implies that the veranda is fixed to completion (probably repeatedly). In that case, the object would be in the accusative in Finnish. The partitive object in Finnish, indicates that we're talking about the act of fixing in its ongoing, incomplete state, which we express with a continuous verb form in English.
I wish people would not make absolute statements about what "we express... in English" because, as we all know, there can be considerable variation in the use of language.
If I am standing watching my neighbour repair the veranda, I might turn to his wife and ask "why does he repair the veranda? ". I find that just as acceptable as "why is he repairing the veranda? ".
Yes, there are absolutely contexts where it is possible to use an ongoing action with the simple present in English.
The difference between the two verb forms is not as large as the difference between the two corresponding object forms is in Finnish. However, this difference is literally vital in Finnish: Ammuin sheriffin means I shot the sheriff (who is now dead), Ammuin sheriffia means I shot at the sheriff (who survived).
As none of the sentences here on Duolingo have any context, we are very strict in maintaining this difference, as it is so important to learn it. When there isn't an object, both are usually accepted.
It's usually not helpful to try to think of any possible context where some translation may work. Anyway, the objective here is to learn Finnish, not "win the game".
I accept completely what you say about the correct use of cases in Finnish. However, the point I and others was making was not about correct Finnish but concerned English translations that should be acceptable.
Yes, the object here is to learn Finnish but many of the people doing so will not have English as their first language so to a degree they are also "improving" their English. It is important we do not mislead them by making incorrect statements about the use of English.
I have absolutely no idea what "win the game" alludes to.