Please don't mess up Latin nouns.
English is a beautiful Mutt language
It adopts everything from tech jargon to Japanese to Spanish and words of practically all languages. But mostly, English is a Germanic language with a particularly large number of Danish, French and Latin words in it. I'm not sure why, but the words that seem to be confused and miss-used the most are Latin nouns; especially Latin singulars and plurals. A LOT of native English speakers butcher these words. University professors, politicians and all sorts of others who should know better use Latin plurals for singulars and vice versa.
Does it matter? Not really to most folks, but to the few like me, who suffered through years of Latin in school it drives us up the wall! I think that's the primary value of high school Latin - making language nit-pickers out of those who take it.
So, if you would like to avoid screwing up your Latin nouns and annoying people unable to get over high school Latin, here are the main culprits:
CRITERION - this is the SINGULAR noun
CRITERIA - this is the PLURAL
PHENOMENON - SINGULAR
PHENOMENA - PLURAL
ALUMNA - SINGULAR, FEMANINE
ALUMNAE - PLURAL , FEMANINE
ALUMNUS - SINGULAR, MASCULINE
ALUMNI - PLURAL, MASCULINE
BACTERIUM - SINGULAR
BACTERIA - PLURAL
There are others, but these are the ones I hear the most used incorrectly; for every time I hear them (or see them) used correctly, I hear them wrong about 4 times. When you see a noun from biology or one that ends in 'us', 'um', 'on', 'i', 'a' or 'ae' it is probably from Latin. It's pretty easy to find the correct plurals and singulars for these words if you have access to a dictionary or google, if you care to check.
If you don't want to bother, that's OK too. Just be forewarned that people around you who suffered through Latin in high school are now suffering again. That might be worth a chuckle at least... :-)