Theory:

1. To make regular nouns plural, add '‑s' to the end.
Example:
i) dog - dogs
ii) river - rivers
2. If the singular noun ends in '‑s', '-ss', '-sh', '-ch', '-x', or '-z', add '‑es' to the end to make it plural.
Example:
i) bus - buses
ii) wish - wishes
iii) lunch - lunches
3. In some cases, singular nouns ending in '-s' or '-z', require that you double the '-s' or '-z' before adding the '-es' for pluralization.
Example:
i) gas - gasses
ii) glass - glasses
iii) fez – fezzes
4. If the noun ends with '‑f' or '‑fe', the 'f' is often changed to '‑ve' before adding the '-s' to form the plural version.
Example:
i) wolf - wolves
 
Exceptions:    
  
i) roof - roofs  
ii) belief - beliefs
5. If a singular noun ends in '‑y' and the letter before the '-y' is a consonant, change the ending to '‑iesto make the noun plural.
Example:
i) puppy - puppies
ii) baby - babies
iii) daisy - daisies
6. If the singular noun ends in '-y' and the letter before the '-y' is a vowel, add an '-s' to make it plural.
Example:
i) ray - rays
ii) boy - boys
7. If the singular noun ends in '‑o', add '‑es' to make it plural.
Example:
i) tomato - tomatoes
 
Exceptions:
 
i) photo - photos
ii) piano - pianos
Important!
With the unique word volcano, you can apply the standard pluralization for words that end in -o or not. It’s your choice! Both of the following are correct:
  • Volcanoes
  • Volcanos
8. If the singular noun ends in '‑us', the plural ending is frequently '‑I'.
Example:
i) focus - foci
ii) fungus - fungi
9. If the singular noun ends in '‑is', the plural ending is '‑es'.
Example:
i) ellipsis - ellipses
ii) oasis -oases
iii) diagnosis-diagnoses
10. If the singular noun ends in '‑on', the plural ending is '‑a'.
Example:
i) phenomenon - phenomena
ii) criterion - criteria
11. Some nouns don’t change at all when they’re pluralized.
Example:
i) sheep - sheep
ii) series - series
iii) species - species