How many nouns use the locative case ending in “y”, and is there any way to anticipate a certain noun would use this as opposed the the more common propositional “е” ending? Is their anything in their etymological origin that would explain it (if one were familiar with it), or is proper usage only learned through study lists or through repeated exposure?
I’ve had enough exposure to the language that “в лесу”, “на полу”, “на мосту” and a few others feel natural and easy. I just came across “все в пуху” which prompts the question of how many other examples are lurking out there.