It's the ones from other languages that are the problem. And since the languages that we've imported wholesale the most words from are Latin and Greek, and since a lot of those words retained their plurals when they were imported, you should know how to pluralize Latin and Greek nouns so you don't look like a complete idiot.
Latin's got more or less three types of nouns--simple, hard, and rare. (These are not the official definitions, but surely those of you who do know Latin will get my drift here.) Latin nouns come in things called "declensions", and have three genders, masculine, feminine, and neuter. (Since English is a genderless language, this doubtless seems vaguely strange to you. There's no real perception of gender attached to these words today. It's a more or less arbitrary division of the language. Some languages go for animate vs. inanimate instead.)
The simple nouns are from the first and second declensions in Latin. Here's how it goes: nouns ending in -a are pluralized in -ae; nouns ending in -us are pluralized in -i(with some exceptions that I'll note later); and nouns ending in -um are pluralized in -a. These are feminine, masculine, and neuter respectively. For example, 'antenna' becomes 'antennae', 'focus' becomes 'foci', and 'medium' becomes 'media'. These are not hard to grasp. The tricky bits are the exceptions.
"Hard" nouns are third-declension Latin nouns. Most of those take a plural in -es, except for the neuter ones, which are pluralized in -a. But what this plural ending is attached to is another story. The easiest of these end in '-is', and you can just remove that and add on '-es'. (So the plural of '-is' is not '-ii', as folk wisdom has it. There is, as far as I know, no basis for this whatsoever.) So, for example, the word 'crisis' is pluralized as 'crises'.
Other hard nouns are trickier, because the normal(or "nominative")case of the noun doesn't contain the root. This root, which is not necessarily obvious and has to be memorized, is what you add the 'es'(or 'a')to. A good example of this is 'index', whose root is 'indic', and hence the plural is 'indices'. Another one, which is a bit misleading, is 'genus'. This is not a 'simple' second-declension masculine noun, as it appears, but a third-declension neuter noun; its plural is thus 'genera'. Similarly, the plural of 'opus' is 'opera'. (Neuter plural words, like opera, data, and media, show a striking tendency to become singularized in English, probably because they look like singular feminine words.)
Finally, the rare ones, fourth and fifth-declension nouns, are just that--rare. Both of these declensions(the fourth in -us and the fifth in -es)have plurals just like the singulars. I can't think of any good examples of fourth-declension nouns in English at the moment(though vacuum is based on another form of such a noun), but common fifth-declension nouns are "species" and "series", neither of which change when pluralized.
Greek nouns are also rare, but you do see a few. The masculine second-declension ones ending in 'os' have mostly been changed into 'us' latin words; they are pluralized in 'oi', but the only extant example of this is the phrase 'hoi polloi'. The neuter nouns in 'on' show up a bit more frequently, as in 'criterion'(plural, criteria). There's also a messy third declension in Greek, but few of its nouns have come across unchanged either. (Some of them are a bit disguised, though. For instance, 'octopus' is one of them; its Greek plural would be 'octopudes', not the faux-Latin 'octopi'. Just play it safe and say 'octopuses', however uneuphonious this may be.)
If you have any doubts about the pluralization of a Latin-type-looking word that's not on here(to forestall a barrage of questions--'penis' becomes 'penes', 'clitoris' becomes 'clitores', and Elvis isn't Latin at all), then email me using the link below and I'll put it up here as well. And try not to be assholes.
Last updated: 1/19/2006
The Den of Ubiquity / Aaron V. Humphrey / email@example.com