"He has one liter of juice left."
Translation:Hänellä on vielä yksi litra mehua.
Same!! It adds extra difficulty since that's not how it translates directly. Also in English there's a slight difference in meaning with "he still has one litre of milk" / "he has one litre of milk left". "He still has one litre of milk" could also mean he's had the same amount all along, not more. But "he has one litre left" really means he's used some amount and there's that amount left. I'm not sure if "hänellä vielä yksi litra maitoa" could have the same two meanings as it does in English but maybe that's why they gave an indirect english translation, if it only has one meaning.
With mehua, oranssia, and juustoa, that -a is the partitive singular ending. You'll eventually also encounter -ta, which is the partitive ending used after consonants, like mies; miestä.
In this sentence, litra makes the following noun partitive. Words indicating an amount, like kilo, paljon, and vähän cause the next noun to be partitive.
As for oikea and väärä, they don't really have an added 'a'. Oikea and väärä are those words' most basic form. The instructive ending -in was added onto them, and that erased the last 'a'.
Oikea and väärä are adjectives, with meanings of 'correct' and 'incorrect', while oikein and väärin are adverbs, 'correctly' and 'incorrectly'.
It gets confusing though, because Finnish uses these adverbs in places we'd use adjectives. Thus "That is correct": 'Tuo on oikein', and "(That's) correct, he's Finnish": 'Oikein, hän on suomalainen'.