There are many words of Latin or Greek origin which end in ‘-us’, however it is not always clear what the plural form is. Do words like ‘hippopotamus’ and ‘octopus’ end in -es or -i? The website About.com explains that it is best to go with hippopotamuses http://homeworktips.about.com/od/plurals/f/hippopotamus.htm, whereas with the word octopus you apparently cannot go wrong if you guess the plural as octopuses, octopodes or octopi http://homeworktips.about.com/od/plurals/f/octopus.htm as they suggest that all of them are correct.
I found the best coverage about this subject on Wiki answers: http://wiki.answers.com/Q/How_do_you_make_words_ending_in_us_into_plural. They explain that you add -es to most words that end in -us; however, “words that come from latin, such as stimulus, nucleus, and radius, still act as they did in latin. That is, they end in ‘-i’: stimuli, nuclei, radii”. There are exceptions like virus where the plural is viruses – the reason for this is explained in the Wiki answers link above.
My thoughts: This was not as straightforward as I thought, although I have learnt a few new rules about awkward plurals today.
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