"A medium cup of juice"
Translation:He kīʻaha wai hua ʻai lōpū
don't remember exactly what the sentence was however, "he" is always "a or an"... "I" at this level of 'ōlelo is a "marker" of a prepositional phrase meaning to, in, of, at, on, etc. If the phrase you refer to begins with "a or an", "he" would be correct translation... if it begins with to, of, at, on, in, etc "i or ma" would be the correct translation... Hope this helps...
The difference is that the sentences that started with "i" ended with something like naʻu, nāu, etc. So it was like a shortened form of "E hāʻawi mai . . . i wai hua ʻai naʻu." Also, they were considered complete sentences (even though shortened). This is NOT a complete sentence, just a phrase: "a medium cup of juice." If you added "naʻu" at the end, the "he" would usually change to "i." Does this help?