An interesting note about the -us -> -i conjugation is that it is only one of several possible -us -> plural forms. The word "
octopus" can be pluralized to
octopi, as the above rule would suggest. However, it can also be pluralized to
octopuses and
octopodes.
This is because English is a bastard language to the fullest extent. Octopus was originally a greek word, and in classical greek, would be pluralized to octopodes. Since this is the original proper pluralization, it is accepted in the english language. The word was assimilated into Neo-Latin, at which point they pluralized it by the -us -> -i rule. When we started speaking English, we decided that we have our own pluralization rule, root -> root+s (or +es). So, speaking proper English, octopus can become octopuses.
I like using octopodes the best, because it just sounds much cooler than either of the alternatives. Unfortunately, it is about as common as hearing octothorpe (which is not in my monster Webster's Encyclopedic Unabridged Dictionary, it's very depressing), so people sometimes look at me funny when I use the word.
It might be interesting to make a psychological evaluation about a person depending on how they pluralize octopus. Those who say octopodes live in the past and are stuck with the way things were. Octopi users are creatures of habit, unable to escape the box of common usage. Those who say octopuses are the forerunners of the degred... err... evolution of the English Language.