"This is not milk."
Translation:ʻAʻole kēia he waiū.
"Milk" is a countable noun in Hawaiian, whereas it's uncountable in English. "Chair", for example, is countable. This is why we say "This is not milk.", but also "This is not a chair." So, because "milk" is countable in Hawaiian you basically need to say "This is not a milk" when translating this sentence into Hawaiian :)
Hmm... I'm not an expert Hawaiian speaker, but I suspect a better explanation is that you always use 'he' for the indefinite as opposed to ke/ka for the definite whether or not a noun is a count noun. Hopefully someone who knows better will way into this conversation about this point.