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
