Is there a rule when to use ι vs η?

Answer 1

There are many rules actually.

«η» is the article for feminine nouns.

When it has an accent «ή» is also the word we use between option just like "or"

The «ι» is placed at the end of neutral objects that receive the article «το» like "αγόρι" and "ποτήρι"

Hope this helped a bit.