Why has 'This is the capital city.' been marked incorrect?
What's wrong with This city is the capital?
Nothing is wrong, it's just the usual problem of a/the translations... Please report :)
Or this is the city capital
That would be Це столиця. This one says "Це місто [є] столиця" = "This city [is] a/the capital", not "This is a/the capital", we just don't say "is", so that's confusing :)