The plural noun is generally formed by adding -s to the singular.
Cow (singular), cows (plural)
Nouns ending in -s, -sh, -ch or -x form the plural by adding -es to the singular.
Class (singular), classes (plural)
Most nouns ending in -o also form their plural by adding -es to the singular.
Mango (singular), mangoes (plural)
Note that a few nouns ending in -o merely add -s.
Dynamo (singular), dynamos (plural)
Nouns ending in a consonant + y form their plural by changing -y into -i and adding -es.
Baby (singular), babies (plural)
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)
Most other nouns ending in -f or -fe add -s.
Chief (singular), chiefs
A few nouns form their plural by making some changes to inside vowels.
Foot (singular), feet (plural)