Should be "i" kēia lā. People may colloquially leave out the "i" when speaking informally, but this is a language "lesson" and should be taught grammatically correct.
Actually, the word i here is not needed because today / kēia lā is the subject of the sentence, and more importantly, it is the thing being described as windy. If you say, It is windy today, then that is He lā makani i kēia lā.
Wouldn't "it is windy today" be something like "He ka makani i kēia lā"?
Does anyone know why it would not also be "This is a windy day?" I'm sure it's just something I am missing but this one is giving me some major trouble understanding. Anyone have any ideas?
It is a question of semantics like Margaret's question. This is a windy day? is then He lā makani kēia. instead. The English word Today is two words in Hawaiian - kēia lā - meaning this day.
It is acceptable as a loose translation because it conveys the same information. This section is also trying to teach specific grammar patterns and rules. So they are pushing a more exact translation. Today is windy would be much simpler than the original prompt - Makani kēia lā.