The Plural

1. add an "s" to the noun:

apple apples
book books
chair chairs

2. after –s, -sh, -ch, -x, -z: add "es":

bus buses
bush bushes
watch watches
box boxes

3. if a "y" is preceded by a consonant: change "y" to "ie" + s:

baby babies
city cities
lady ladies

4. if a "y" is preceded by a vowel: just add an "s":

boy boys
day days

5. change "f" to "v":

wife wives
knife knives
shelf shelves
wolf wolves
life lives
calf calves
loaf loaves
leaf leaves
thief thieves

exceptions:
roof roofs
chief chiefs
cliff cliffs

6. irregular forms:

man men
woman women
child children
mouse mice
foot feet
tooth teeth
goose geese
louse lice
ox oxen

7. note:

tomato tomatoes
potato potatoes

but:
piano pianos
photo photos
Eskimo Eskimos

8. these words only take the plural form:

trousers
jeans
pants
tights
scissors
glasses (optical glasses; sunglasses)

9. these words only take the singular form:

fish
sheep
deer




Source : Grammar Tutorial Practice makes perfect