First time I have seen this meaning of devoir. Is this a common usage of devoir?
We weren't. Doulingo does not suppose we already do know the meaning of the given exercices but supposes that we do our best try; reflect upon the result; ask each other, books or other resources for explanations; and repeat the unit until we master it.
The "v" in "doivent" is audible, so you have to infer that it is third person plural. With third person singular, you would either hear no consonant at the end of "doit", or you would hear the "t" (if "doit" was followed by a vowel). A lot of times you have to listen for liaisons in order to determine the meaning correctly. http://french.about.com/library/pronunciation/bl-liaisons.htm
You could tell from the way doivent was pronounced. That's what made me figure it out. The v wasn't silent.
It has now. Duolingo does not always first present us with hints to the meaning of the words, or the correct choice of meaning, but gives us different kind of exercices and expects us to make our best try, reflect upon the result, ask each other, books or other resources for explanations and then repeat the unit until we master it.
Is the use of devoir here a matter of expression? I thought it only means "have to."
"To owe" is the standard meaning for "devoir", when it is used transitively - that is, when the verb has an object: "I owe the bank $500", "they owe you a meal" When it is used as an auxiliary verb - when it is paired with another verb - then it means "have to" or "must".
Duolingo lists 'They owe y'all a meal.' as a correct translation. Isn't 'y'all' slang?
I would say it is non-standard rather than slang, it may depend on the region though.
Well, we could perhaps compromise on "extremely non-standard". I believe it would be a disservice to learners of English (of whom there are many here) to give the impression that it was in any way standard English.
Why is it wrong to say "They have to pay you a meal"? I mean, Duolingo offers "have to pay" as a translation for devoir..