"A cup of juice for me."
Translation:I kīʻaha wai hua ʻai naʻu.
Barbara, this "sentence" is actually just a phrase, although the meaning is clear and it is used in English like this too. There is no verb in this phrase. If you said the formal sentence, it would start with "I want...". So, if you imagine starting the whole sentence with "Makemake au", you would naturally have "i kī'aha..." next. "He" gets you the English idea of "a/an" and requires the verb "to be" when translated into correct English.
To boy questions, I don't know the answer, and I want to. But maybe they're interchangeable. I haven't tried it, and if you have and tx're not, then maybe Duo made a mistake. It could always be some rule we don't know about, though, but it's looing more and more like they're interchangeable. It could be something with the "for me" part of the sentence affecting the beginning, but I don't know what the rule would be or if that rule even exists. Just something to think about.