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