Ezra Pound
Forums. Some words maintain their Latin declensions (at least the distinction betwixt nominative singular and plural), some don't -- the only difference I can find is common usage. In the interest of not wasting time explaining what I meant, I use forums.
"Datums" will never show up, since some people erroneously use data for singular and plural.
I agree. I don't think I've ever heard anyone say "datum" outside of discussion about plurals of Latin words.
I usually say "forums" to avoid confusing my friends.
This reminds me of a chemistry lesson I had. The teacher wrote "aquaria" on the board, and quite a lot of people in the class had no idea what he meant by "aquaria", so he had to explain that "aquarium" is a Latin word and therefore becomes "aquaria" in plural form. That teacher did Latin when he was at school, and he expected everyone to know some Latin, so he wasn't very happy about that. The trouble was that most of my classmates had enough trouble understanding him even if he didn't use these words, but that's a different story on its own.