Group one words, which end with 'a' are usually feminine. Group two words, ending with 'us' are usually masculine, although can be neuter. In group 3, it can be any, but the neuter ones end with 'a' if plural. Not the best help, but what I know!
I don't know if this is still active, but I study Latin (unfortunately) and might be able to help.
Fey he seemed, or the battle-fury of his fathers ran like new fire in his veins, and he was borne up on Snowmane like a god of old, even as Oromë the Great in the battle of the Valar when the world was young. - The Lord of the Rings: Return of the King
When we are children we seldom think of the future. This innocence leaves us free to enjoy ourselves as few adults can. The day we fret about the future is the day we leave our childhood behind. — Patrick Rothfuss, The Name of the Wind
