The correct sentence would be "Les hommes boivent du lait."
Des is used when the object is plural in French terms. Ex: "Je mange des épinards.", meaning I eat some spinach.
Des is also used when an object is countable, but in an indefinite sense. Ex: "J'ai des clients intelligente.", meaning I have some smart clients.
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The men drink milk and men drink milk (the other translation supplied) is ambiguous. "The men" would indicate something like "those men over there", versus men drink milk, as in all men do so (which is illogical, too). Is there not better way to define this in french grammar?
Basically, you can think of <<du>> meaning "some" (however, don't think of this as a literal translation because in the sentence <<Je bois DU cafe>>, for example, you don't have to translate it to "I drink some coffee", you can just say "I drink coffee"). Le/la/l' literally translate to "the". So think if you were saying the sentence "I drink coffee" but you were talking about coffee in general, not specific coffee, then you would say <<Je bois du cafe>>, meaning "I drink coffee", as opposed to if you were saying "I drink THE coffee", which would be <<Je bois LE cafe>>.
Here is the conjugation of the verb "boire":
"du" is an indefinite article used for uncountable nouns, such as "temps" (time), "lait" (milk), "pain" (bread), etc, etc...
"du" can also be a mandatory contraction of "de + le" as in "Jai besoin du vélo de mon voisin." ("I need my neighbour's bike."). But that's not the case in this exercise.
To learn more about French articles, you can start here:
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