The plural noun is generally formed by adding -s to the singular.
Cow (singular), cows (plural)
Boy, boys
Girl, girls
Book, books
Tree, trees
Dog, dogs
Pen, pens
Nouns ending in -s, -sh, -ch or -x form the plural by adding -es to the singular.
Class (singular), classes (plural)
Brush, brushes
Kiss, kisses
Match, matches
Dish, dishes
Most nouns ending in -o also form their plural by adding -es to the singular.
Mango (singular), mangoes (plural)
Hero, heroes
Potato, potatoes
Volcano, volcanoes
Note that a few nouns ending in -o merely add -s.
Dynamo (singular), dynamos (plural)
Piano, pianos
Photo, photos
Logo, logos
Kilo, kilos
Nouns ending in a consonant + y form their plural by changing -y into -i and adding -es.
Baby (singular), babies (plural)
Lady, ladies
City, cities
Story, stories
The following nouns ending in -f or -fe form their plural by changing -f or -fe into v and adding -es.
Thief (singular), thieves (plural)
Wife, wives
Leaf, leaves
Half, halves
Self, selves
Calf, calves
Loaf, loaves
Knife, knives
Elf, elves
Wolf, wolves
Shelf, shelves
Most other nouns ending in -f or -fe add -s.
Chief (singular), chiefs
Gulf, gulfs
Safe, safes
Proof, proofs
A few nouns form their plural by making some changes to inside vowels.
Foot (singular), feet (plural)
Man, men
Woman, women
Tooth, teeth
Mouse, mice
Goose, geese