"This milk is new."
Translation:هٰذا الْحَليب جَديد.
The noun and adjective are different definitives. Pretty sure it should be either both ال or no ال
The case you are talking about is the "attributive adjective" case. However in a predicative sentence where someone is telling a fact, the adjective and the noun before it do not mimic the same attributions, like being definite.
Here is an article which I hope it helps (done it 2 nights ago): https://forum.duolingo.com/comment/33483445
هذا الحليب جديد = "This milk is new."
هذا الحليب الجديد = "This is the new milk."