I appreciate DuoLingo launching Latin, but it's a shame the speakers (including the male speaker) possesses such a strong Anglo-Saxon/American accent when pronouncing most of the words. He blows his Ts, he pronounces the vowels like an American would pronounce them...and the same happens with his "D"s. The way he says "Quomodo" is just not right... As a speaker of five major Romantic/Latin languages (Portuguese, Spanish, Italian, French, Catalan), I must say he does not sound natural at all. I know we cannot testify 100% how Latin sounded exactly, but it definitely did not have those hints of Anglo-Saxon sounds, otherwise all these other languages would have evolved quite differently..
Interesting. English names sometimes have shortened or alternate versions, such as Billy for William, Jeffy for jeffrey, jimmy for james. Etc. Not the same sound but similar and slightly similar concept, but denoting much more of an intimate relationship. But no official vocative form. Never heard that phrase.
So does Welsh -- although it's not used with personal names . As usual with grammatical distinctions in Welsh, it's the start of the word that's affected.
Pawb = everybody / Helô bawb! = Hello, everybody!
Boneddigion = gentlemen / Bore da foneddigion = Good morning, gentlemen
 At least, not in modern Welsh. You can still say A thithau Frwtws? -- Et tu Brute? :)
Portuguese one do have https://www.soportugues.com.br/secoes/sint/sint23.php
I am getting a mixed message here: are initial h characters pronounced or not? With some recordings, with "habeo," it was. Here it is not. I feel the silent h would be more appropriate in modern times given other Romance languages, but older Vulgar Latin had the h pronounced originally, so which are we using here?
Your constructive criticism is very helpful. I especially appreciate how you offered advice on how an improvement could have been made, instead of just insulting the people who are working hard on something that's still in beta.
Simply offering an insult without advice when you know more than someone else would be a dick move.
From the standpoint of a native speaker of Canadian English, I would say that the "doing" certainly isn't mandatory, but also isn't completely redundant; it subtly changes the perceived intentions of the question. It can either be added to make the question more informal and more strongly imply that one actually wants an honest answer to the question (and aren't asking simply as a formality), or it can be added to indicate one is checking on the progress of an ongoing action, event or personal state and the questionee's feelings towards the aforementioned.
I'm not sure I even understand the question, but assuming I do, it's just that you NEVER translate the vocative case into English, for the simple reason we don't have one; we don't go around changing people's name-endings to show we're speaking to them. But once you understand that Latin does, and Marcus becomes Marce if you're speaking to him, that's it! Marcus in English is always Marcus. We don't muck about with the ending (except for possessives).
That is because it is wrong. We don't have a vocative case in English (the thing that, in Latin, turns 'Marcus' into 'Marce'). So, when we translate, we should give him his "normal" name of Marcus, not preserve a case change that is required in Latin, but non-existent in English.
I think I am a little cross with the Latin course. I appreciate Marce is a form of Marcus - but surely this should be a "you have made a typo " instead of saying "this is wrong." There is some flexibility in spelling other words wrong but zero when it comes to people's names. this is somewhat infuriating. If I put habe instead of habes - it'll say "you've a typing mistake"
But the ending in this instance is important to indicate the case. If you get it wrong, how is DL to know whether it's a "simple typo", or you have not understood the vocative case? In fact, I myself have no idea from your post whether you don't understand/recognize the case, or do, but made a typo. So how is an algorithm supposed to work it out? Although it's harsher, I think it's more instructive for DL to assume you don't understand the case, rather than pass it as a mere "typo".
You are not translating a name, you're getting it wrong. His name is Marcus. See the comments above: there is a special case in Latin, called the vocative, which changes the ending, when you specifically address someone. But his name is still Marcus; the change doesn't mean he's suddenly changed his name, it's just used as an indicator that he's the person being addressed.