Theory:

Apophonic plurals:
The plural is sometimes formed by simply changing the vowel sound of the singular (these are sometimes called mutated plurals).
Example:
i. foot - feet
ii. goose - geese
iii. louse - lice
 
1. Irregular plurals from Latin and Greek.
Example:
i. formula - formulae/formulas
ii. supernova - supernovae
 
2. Final 'ex' or 'ix' becomes '-ices'
Example:
i. index- indices
 
3. Final 'um' becomes '-a':
Example:
i. agendum - agenda
 
4. Plural possessive noun: plural noun with a possessive ending to show ownership.
Example:
i. We got our cats' beds from the store.
ii.week's vacation