Now, in general, we know that to make the plural form of a noun we add either an 's' or an 'es' to the singular. But we have some rather special nouns that don't follow this rule:
Singular Plural
man men
child children
woman women
ox oxen
mouse mice
louse lice
goose geese
tooth teeth
foot feet
cod cod
trout trout
salmon salmon
sheep sheep
deer deer
oasis oases
radius radii
bacterium bacteria
antenna antennae
focus foci
syllabus syllabi or syllabuses
terminus termini or terminuses
alumnus alumni
This is not a complete list, but it lists some of the more common plurals that are a bit strange
Singular Plural
man men
child children
woman women
ox oxen
mouse mice
louse lice
goose geese
tooth teeth
foot feet
cod cod
trout trout
salmon salmon
sheep sheep
deer deer
oasis oases
radius radii
bacterium bacteria
antenna antennae
focus foci
syllabus syllabi or syllabuses
terminus termini or terminuses
alumnus alumni
This is not a complete list, but it lists some of the more common plurals that are a bit strange