Today I'm going to discuss a few words that don't form the plural by adding the letter 's'. These words come mainly from foreign 'imports', although a lot are very old. Some people are confused by these words and use the plural as a singular.
So here we go.
Singular: Bacterium Plural: Bacteria
Singular: Phenomenon Plural: Phenomena
Singular: Medium Plural: Media
Singular: Datum Plural: Data
Singular: Criterion Plural: Criteria
Singular: Cactus Plural: Cacti
Singular: Fungus Plural: Fungi
Singular: Stadium Plural: Stadia
Singular: Nucleus Plural: Nuclei
Singular: Syllabus Plural: Syllabi
Singular: Focus Plural: Foci
Singular: Thesis Plural: Theses
Singular: Crisis Plural: Crises
Singular: Index Plural: Indices
Singular: Appendix Plural: Appendices
It is becoming more acceptable to hear 'stadiums', 'syllabuses' and 'indexes', although they grate on me, personally.
Now for some that don't change for the plural.
fish (although the word 'fishes' can be used if referring to a number of different types of the creatures. e.g. There was a great variety of fishes swimming around on the reef.)
Now what about those that are completely different in the plural? Here we have the following:
Singular: Child Plural: Children
Singular: Man Plural: Men
Singular: Woman Plural: Women
Singular: Mouse Plural: Mice
Singular: Goose Plural: Geese (N.B. The plural of 'mongoose' is not 'mongeese', but 'mongooses'.)
Singular: Die Plural: Dice
Singular: Foot Plural: Feet
Singular: Louse Plural: Lice
Singular: Ox Plural: Oxen
Singular: Person Plural: People
Singular: Tooth Plural: Teeth