If there is no "il/the" in the Italian sentence, there will be no "the" in the English sentence. If there is a "the" in the English sentence, there will be an "il/the" in the Italian sentence. The problem for English speakers is if there is an "il/the" in the Italian sentence. What do we do? As in English, if the Italian sentence is about a specific thing, like the glass of milk on the table that we're talking about, you would use "il/the" just like we do in English. If it were about milk in general, the Italians many/most? times just throw an "il/the" in where we do not in English. But, the Italian sentence is correct without the "il", as it is here.
That is how it is being taught here by Duolingo, and they will accept both ways as correct. Some Italian speakers in these threads have said that they use the definite article when we would not in English, like in this sentence. There is a man who posts a lot, though, who strongly disagrees. I'm not sure if his native language is Italian or not. I have tried to research this question on line but have not been able to find an answer to this specific question. I guess consulting a text book that teaches Italian is the next logical step in getting a definitive answer to this question which, in my mind, remains unanswered.