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?
Best Answer
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.