What a surprise! Here is no 'i ka lumi', only 'ma ka lumi'. It seems like a Duolingo's error. Or some explanation is really needed.
why is the "i" now a "ma"... if you cannot take time to explain YOUR purpose, then allow i and ma to be equal as is the case in all beginning ʻōlelo classes...
Any historical information on the why fork - `o - takes "ke" instead of "ka"?
I understand the KEAO rule, but ʻō does start with an ʻokina, not just an o. Does that not change things?
There are exceptions to the rule, this is one. It seems that a lot of the kitchen/eating related words are exceptions: ʻō, puna, pā, pākaukau are all exceptions. Pahi isn't, but I think the word is more for the weapon than the eating utensil.
"is in" should be accepted as "i ka"