I've taken a few notes about the words end with "um". I once read an article which said that you can't find a regular rule on how to form plurals about these kind of words becaues they are foreign wrods which were imported in English. There's only tendencies which means you just need to follow the majority when forming the plural.
The following are the words I took down in my notebook. I think you guys can give me more words. Thank you very much !
quorum quorums
momentum momentums
rectum recta
forum forums
bacterium bacteria
conundrum conundrums
ultimatum ultimatums
datum data
medium media
consortium consortia
curriculum curricula/curriculums
memorandum memoranda/memorandums
symposium symposiums/symposia
