Okay if This is not milk is he then why isn't this sentence Inu kāua i HE waiū?
Because kāua drink milk, in definitive. The sentence wouldn't make any sense if milk was indefinitive.
Doesn't the subject go at the end? I got it all twisted up with "Inu ka waiū i kāua."
Hawaiian is Verb-Subject-Object order. The i comes before the object to mark it.
Why the difference between ka and ke in this sentence.
The usage of ka vs ke is dependent on the noun it precedes. In general, ke is used with nouns beginning with k, e, a, or o, while ka is used with other nouns. Some exceptions exist - just keep an eye out for them