Singular -us becomes -i in the plural (pronounced as the word “I”)
- cactus - cacti
- fungus - fungi
- syllabus - syllabi
Singular -a becomes -s or -ae
- antenna - antennas (common sense) or antennae (referring to insects’ sensory organs)
- formula - formulas or formulae (pronunced as the “e” in “me”)
- larva - larvae (pronounced as the “e” in “me”)
- nebula - nebulas or nebulae (in astronomy)
- vertebra - vertebras or vertebrae (pronunced as the “ey” in “hey”) (vertebras is also correct)
Note. As you can see above, it is pretty safe to use the regular plural for these words; however, sometimes many people mistakenly use the irregular plural for the singular (e.g., saying “vertebrae” instead of one vertebra).
Singular -ix becomes -ices or -es
- apex - apices or apexes
- appendix - appendices or appendixes
- index - indices or indexes
- vortex - vortices or vortexes
Singular -on or -umbecomes a
- bacterium - bacteria
- criterion - criteria
- datum - data
- erratum - errata
- medium - media
- stratum - strata
Singular -on or -umbecomes a or -ums
Note. On the list below, the more commonly used plural form is given first.
- addendum - addenda or addendums
- aquarium - aquariums or aquaria
- atrium - atria or atriums
- candelabrum - candelabra or candelabrums
- curriculum - curricula or curriculums
- memorandum - memorandums or memoranda
- millennium - millennia or millenniums
- moratorium - moratoriums or moratoria
- podium - podiums or podia
- referendum - referenda or referendums
- spectrum - spectra or spectrums
- symposium - symposiums or symposia
Singular -is becomes -es (pronounced as the “ees” in “fees”)
- analysis - analyses
- basis - bases
- crisis - crises
- diagnosis - diagnoses
- emphasis - emphases