Add -S after most nouns.
1 dog, 2 dogs, 3 dogs

Add -ES after words ending in S, CH, SH, X, or O.
1 bus, 2 buses
1 match, 2 matches
1 bush, 2 bushes
1 box, 2 boxes
1 mango, 2 mangoes

Change consonant Y to IES
1 puppy, 2 puppies

Don't change vowel (AEIOU) Y to IES
1 boy, 2 boys

Change F to VES
1 leaf, 2 leaves
1 thief, 2 thieves

Some nouns do not follow the rules.
1 fish, 2 fish
1 person, 2 people
1 man, 2 men
1 woman, 2 women
1 ox, 2 oxen
1 child, 2 children
1 tooth, 2 teeth
1 foot, 2 feet
More Exceptions

Most dictionaries will include irregular plural forms.
dictionary example
Dictionary.com - is an excellent free online dictionary.

Practice - The box will turn green when you spell the plural form correctly.

  1. cat -->
  2. bird -->
  3. tomato -->
  4. boss -->
  5. candy -->
  6. toe -->
  7. fax -->
  8. half -->
  9. lion -->
  10. woman -->
  11. deer --> *Irregular, check the dictionary.