To me it sounds fine. Like if your child said at breakfast with friends - "What are you putting on their cereal?" You might respond - "Oh, honey, they drink (a) different milk" as you pour them soy milk. For me I can usually find a question that will give this answer - it's just formulating it correctly.
In any case, it gives us a chance to exercise our new adjective "разный" with a neuter noun.
The sentence is grammatically awful. It's how a Russian tourist might describe purchasing skim versus whole milk. "My family, they drink different milk. Give me different milk!"
A modest improvement, "They drink a different milk", has been proposed and ignored too many times to count. I will abandon my diet and celebrate with a glass of whole milk if the DL Russian team ever corrects this.
"Разное" means "different from each other". I.e. one of them drinks, say, goat's milk, the other one drinks cow's milk, so together they drink different milk. If it was a countable noun we'd use plural here. "Они носят разные рубашки" - "they wear different (from each other) shirts". The suggested "a different milk" might be correct English, but it's an incorrect translation, because it suggests that they drink the same milk together, when the point of Russian sentence is that they don't.
"A different milk" would be "другое молоко".
It's amazing that English doesn't distinguish between two such different concepts.
I understand the distinction. That should be evident from my example of skim versus whole milk.
The problem is "they drink different milk" is no less ambiguous than "they drink a different milk" but is decidedly improper English.
Let us reconsider our hapless Russian tourist: "Hey, waiter. There was fly in milk. They drink different milk." Or, "Milk taste sour; they drink different milk." Same kind of milk, but nevertheless "different milk".
To summarize: omitting the indefinite article does nothing to imply the speaker is referring to the type of milk but does betray a fundamental misunderstanding by the speaker of proper English grammar. The preferred answer should be "they drink a different kind (or type) of milk" and an acceptable answer should be "they drink a different milk".
I don't think you do understand, actually. I initially added the answer with the indefinite article, but on a second reading, it really is incorrect. "They drink a different (kind of) milk" means that A and B together drink some kind of milk that is different from other kinds of milk, but both A and B drink this different milk, they drink the same kind of milk. But the Russian sentence actually means that A and B drink different kinds, i.e. they do not drink the same kind as each other. So the acceptable translations would be "they drink different milks" and "they drink different kinds of milk".
Got it. Unfortunately, not one of DL's now-accepted English translations rules out A and B drinking the same kind of milk.
The bane and beauty of nearly any declarative English sentence is that it is prone to multiple interpretations. Duolingo's suggested answers are no exception.
I've been a commercial lawyer for 33 years. Like most such lawyers, I've made a living writing incredibly long sentences that, by virtue of their many qualifiers and legalese, pre-empt unintended interpretations. Yet we still wind up in court, arguing about -- what else? -- what a contract or document actually means.
If you intend to rule out the collective third-party "they" (parties A and B) drinking a different kind of milk than C (C being the first-person group or speaker), you need a longer sentence. Period. I suggest, "They are each drinking a different kind of milk" or "They are drinking a different kind of milk than each other." Both are long-winded, but that is the nature of the beast.
Lest this not be sufficiently evident, note that the shortest English translation of War & Peace on Amazon is 587,287 words, yet the Tolstoy original contains only 188,088. English sentences cannot, as a rule, attain the same level of precision as Russian ones without a series of qualifiers. In the case of War & Peace, the required ratio of additional verbiage was 3:1. By that standard, my proposed translations are a bargain.
Avoiding ambiguity isn't necessarily feasible. The accepted answers are certainly ambiguous, but they're also natural ways to express the meaning of the Russian sentence so they have to be accepted. Your more verbose sentences are also accurate translations, I'll add them as accepted answers.
I am grateful Duolingo has adopted this more expansive approach.
Until 16:20 EDST 2 July 2018, the required translation of "Они пьют разное молоко" was "They drink different milk," which had the twin defects of being hopelessly ambiguous and improper English. This generated a firestorm of criticism because the equally ambiguous but grammatically correct "They drink a different milk" was rejected ... until July 2, 2018 -- fully two years after the controversy arose.
I am overjoyed that genuine English has finally been embraced. Memorizing awkward, pidgin-English expressions to complete a Russian-language exercise was, to put it mildly, frustrating. Thank you so much for all the energy and heart you and fellow DL volunteers have invested in this course. It has become truly excellent because of it.
If the Russian means that, among themselves, they drink various types of milk, could we not say "They drink various milks" as an acceptable translation, or something similar?
I think what is making things difficult for the English is that "different milk" is read as only one type of milk by default, while if we wanted to specify that there are several kinds of milk involved, we would say precisely that: "kinds of milk", "types of milk", or something similar.
Maybe it would help if we had some more examples in which "разный" is used in the singular. My dictionary only gave examples in the plural.
What did you try to answer? This sentence means that they drink different milk from each other. Sentences like "they drink a different milk" won't be accepted, those would translate "они пьют другое молоко".
I'm also not sure how you even saw this sentence. We actually decided to remove it from the course this morning.