What are Irregular Plurals?
English grammar generally pluralizes a word by tacking -s or -es onto the end of it, but there are plenty of words that buck this; some noun plurals look extremely different from the singular.
Latin and Greek Origins
Irregular plurals of noun difficulties can be partly blamed on English’s habit of importing from other languages. Two that are extensively borrowed from are Latin and Greek. Here are some words of Latin or Greek origin with their strange plurals (some have additional plurals, but these are the most irregular). You may recognize some by the plural rather than the singular, such as “algae” and “bacteria.”
Phenomenon, phenomena
Focus, foci
Bacterium, bacteria
Cactus, cacti
Fungus, fungi
Medium, media
Stimulus, stimuli
Larva, larvae
Nucleus, nuclei
Hippopotamus, hippopotami
Vertebra, vertebrae
Alga, algae
Radius, radii