It accepted "all or nothing" as a translation. Is this a common expression in Russia as it is in America?
(Edit: By which, of course, I mean: Is the statement "всё или ничего" as commonly used as the American translation, or is this here simply to show how these words counter one another.)
There is a rule prescribing that the adverbs "ничего" and "сегодня" (derived from "сего" + "дня"), as well as the genitive and accusative cases of masculine and neuter adjectives, participles, and adjectival pronouns ending on -ого/-его, retain the "г" letter despite the latter being pronounced as "в".
So in writing we get: его, моего, всего, чего, одного, хорошего, большого, чистого.
But those are pronounced as ево, всево, чистава etc.
As for why the consonant sound changed in the first place - that's a good few centuries of sound change at work here. The most widely accepted theory, proposed by Mitrofan Kolosov and Alexei Sobolevsky, is that the voiced velar stop (the hard 'g') between two labialised vowels gradually turns into a voiced velar fricative, thus marking a natural human tendency to articulate less in daily speech. This sound ('ɣ') is now found in some Southern and Northern dialects of Russian, and can even occur in Standard Russian when a short word like "угу" or "ого" is spoken in an offhand manner.
Eventually, this sound weakened to the point of complete erosion, leaving us with an ending that sounded like "-оо/-ео" (which is also found in some Northern dialects), which was then broken up by a voiced labiodental fricative (the 'v' sound) to make it more intelligible.
Nothing fancy, that's just the way Duolingo renders the Russian italics (i.e. text in between single asterisks, like *тогда* becomes тогда) - which I use to differentiate the Russian bits from the English bits in my comments whilst cutting down on quotation marks.
You're probably the first person to have commented on that cursive script in a positive context: generally people complain that it only makes comments more difficult to read because of how different letters "д", "т", "г" look. ;j