"Today is a beautiful day."
Translation:He lā nani kēia lā.
Today is actually just "kēia lā". The "i", when used with "kēia lā" marks it as a time stamp - the time frame when something happens. In this case, you are not saying that today is when the beautiful is happening (though that is true). In this case you are specifically saying that today is the thing that is beautiful.
In English we don't mark time stamps differently from subjects so "today" doesn't change between "today is beautiful" vs. "today it is beautiful". But in Hawaiian, you have to add the "i" marker when using it as a time stamp instead of the subject. Since we don't have to do that in English, we have to put a little extra attention to whether "today" is being used as a thing or as a time stamp when saying it in Hawaiian.