I see what you're saying, but what I'm saying is in English we don't say "the soup is with milk". If you do, you're meaning that those two things are together: "The dog is with me", e.g.
However, this structure is quite common in Turkish, and I don't think "with" is a good translation. Another example: börek peynirlidir. But in English we wouldn't say "the börek is with cheese" Instead, we'd say "it's cheese börek" or "the börek is made with cheese".
I guess it's a quite literal translation (since "li" is usually "with"), but not something most native English speakers would say.
As discussed in the sentence about "cake with milk", I agree that "with" just doesn't work in English. It could mean "contains" or it could mean "accompanied by" - where the milk is separate. Since these are two separate meanings in Turkish and it's totally awkward and vague in English, I was thinking "contains" might be a good translation for this suffix. At least the meaning is unambiguous
Yes, people do say it, and it is understandable. Are you a native English speaker? I hear Turks say stuff like that, and it doesn't sound like good English to me. Just like I say perfectly understandable things in Turkish all the time that aren't the way Turks say it.
I don't understand why it doesn't accept 'the soup contains milk' when that's what this sentence seems to mean, having read the other comments. Why force us to phrase it in such an unnatural way? It doesn't help learning, it actually makes it harder to understand the phrase and when it would be used. If a Turkish person would say this to mean that the soup contains milk, then we should learn that, not a literal translation that doesn't really mean anything.
"Çorba sütlüdür." Translation: The soup is with milk.
"The soup has milk in it." Duo accepted this English answer.
Why is 'soup with milk' wrong? How does one say 'soup with milk'?
Soup with milk. "Çorba ile sütlü."
Your answer suggests that you want, "soup with milk."
The Turkish question is saying, "the soup is already with milk."
The "-dür" suffix is stating a fact in the Turkish question. It can also be used for suggestions.
Duo is fickle.