"Today is a calm day."
Translation:He lā mālie kēia lā.
That is a looser translation of it. However, the word today is the subject of the sentence. Putting i kēia lă means that it is no longer the subject of the sentence, just a temporal prepositional phrase.
He lā mālie i kēia lă would translate as It is a calm day today.
Technical but I think that is part of what they are trying to teach in this section.
Wow, that really is a fine point. But I see your point. I repeated both translations out loud, and they both feel the same to me. You are right, though. "La" is the subject. "I keia la" moves "la" into the prepositional phrase, there by requiring a restructuring to "He malie la," I think.
I may be missing something ... prepositional phrases and other grammatical elements are about 40 years away from me, and 'o, o and i are leaving me completely mystified--although I now get that 'o identifies the subject. Is there a place on this website where this is laid out and explained in detail?