English loves to bring in words from other countries. We have adapted many into English. This can be a problem for foreign learners because they might be too similar, or too different. Here is a short list (this is not all of them) of words that have unusual plural forms because they come from another language. Today's examples are either Latin or Greek.
Singular Plural
oasis oases
crisis crises
phenomenon phenomena
cactus cacti
bacterium bacteria
criterion criteria
formula formulae
nucleus nuclei
fungus fungi
appendix appendices
Please take a moment to look up these words (and their pronunciation) in the dictionary--there is an online dictionary that will pronounce the words if you want to hear them (howjsay.com)
