When do I use -i for expressing the plural of a word?

I've never been certain of the rules surrounding the use of the -i suffix for pluralizing a word. I had thought that it was used for any word whose singular ended in an 's', but that doesn't appear to always be true.

For example, the plural of octopus can be written octopi. But the plural of chorus seems to always be choruses, never chori.

When is it proper to use an -i for pluralization?

Use "-i" for plurals when the word is borrowed from a Latin word that used "-i" for plurals. Note that in Latin not all words that end in "-us" are made plural by changing the "us" to "i". For example, in Latin the plural of "locus" (place) is "loci", but the plural of "fructus" (fruit) is "fructus".

Octopus comes from Greek and not Latin, and so does not follow the Latin rules.

I've never been certain of the rules surrounding the use of the -i suffix for pluralizing a word.

There are many nouns whose plurals end in -i, and contrary to popular misconception, quite a few of them did not get there from a -us singular.

Here from the OED is a semi-random assortment of nouns whose plurals end in -i, with their singulars included:

abacus > abaci, agape > agapai, albergo > alberghi, alumnus > alumni, aptychus > aptychi, bacillus > bacilli, bajocco > bajocchi, bandit > banditti, blin > blini, caduceus > caducei, calzone > calzoni, capo > capi, castrato > castrati, casus belli > casūs belli, centumvir > centumviri, cognoscente > cognoscenti, concerto grosso > concerti grossi, conoscente > conoscenti, conversazione > conversazioni, denarius > denarii, dilettante > dilettanti, diplococcus > diplococci, divertimento > divertimenti, duumvir > duumviri, frate > frati, fungo porcino > funghi porchini, fungus > fungi, genius > genii, glissando > glissandi, graffito > graffiti, humerus > humeri, ichthyosaurus > ichthyosauri, intaglio > intagli, intermezzo > intermezzi, isthmus > isthmi, kernos > kernoi, laika > laiki, leu > lei, libretto > libretti, maestro > maestri, mafioso > mafiosi, Magus > Magi, miles gloriosus > milites gloriosi, modello > modelli, modulus > moduli, modus > modi, mondo > mondi, monsignor > monsignori, niello > nielli, nucleus > nuclei, obelus > obeli, oboe d’amore > oboi d’amore , onager > onagri, paparazzo > paparazzi, papyrus > papyri, phallos > phalloi, phallus > phalli, ragazzo > ragazzi, ranunculus > ranunculi, ricercar > ricercari, ritardando > ritardandi, samurai > samurai, Scomber > Scombri, scudo > scudi, septemvir > septemviri, sestiere > sestieri, sforzato > sforzati, siglos > sigli, siglos > sigloi, squadrist > squadristi, squalus > squali, strategos > strategoi, strategus > strategi, suggestio falsi > suggestiones falsi, syllabus > syllabi, talus > tali, tarantato > tarantati, tempo > tempi, terminus > termini, thalamos > thalamoi, thalamus > thalami, thesaurus > thesauri, thymus > thymi, topos > topoi, torus > tori, triumvir > triumviri, udarnik > udarniki, umbilicus > umbilici, uomo universale > uomini universali, uomo > uomini, uterus > uteri, vagus > vagi, vaporetto > vaporetti, viale > viali, volcanello > volcanelli, zeppole > zeppoli, and zita > ziti.

Similarly, there are plenty of -us nouns that do not go to -i plurals. Many of these are from the Latin 3^rd declension, and some are from its 4^th. Others aren’t from Latin nominative singulars at all, like onmibus, a dative plural, and ignoramus, which was a verb in the 1^st person plural present indicative active (the Romans were really tense, you know).

Here are a few from the OED:

apparatus > apparatus, callus > calluses, cantus > cantus, chorus > choruses corpus > corpora, crus > crura, genus > genera, glomus > glomera, hiatus > hiatus, hippopotamus > hippopotamuses, ignoramus > ignoramus, isthmus > isthmuses, logodaedalus > logodaedale, magnum opus > magna opera, meatus > meatus, nautilus > nautiluses, octopus > octopodes, omnibus > omnibuses, pectus > pectora, planctus > planctus, plexus > plexus, prospectus > prospectus, sinus > sinuses, status > status, status > statuses, subgenus > subgenera, summum genus > summa genera, urus > uruses, and Venus > Veneres.

Hm, I’d thought Venus was a rare 2^nd declension feminine like humus. Guess not.

As you can see, the only reasonable answer is to look it up.

(Insert mumbles about hapax legomenon > hapax legomena.)

EDIT

And before it comes up, the plural of virus is simply viruses in English.