Существительные, оканчивающиеся на -y/-o/-f

Forming the plural of nouns that end in -y, -o, and -f/-fe follows specific rules, often with a number of common exceptions.

The rule for nouns ending in -y depends on the letter that comes before it.

Правило	Единственное число	Множественное число
If a noun ends in a consonant + -y, change -y to -ies.	city	cities
If a noun ends in a vowel + -y, simply add -s.	day	days

For nouns ending in -o, the plural form can be either -s or -es, and sometimes both.

Правило	Единственное число	Множественное число
Add -es if the noun ends in a consonant + -o.	hero	heroes
Add -s if the noun ends in a vowel + -o.	radio	radios
Many nouns ending in -o have an irregular plural form, or both forms are acceptable.	potato / piano	potatoes / pianos

Nouns of foreign origin and some common nouns only add -s.

For nouns ending in -f or -fe, the plural is usually formed by changing the ending to -ves.

Some nouns ending in -f or -fe simply add -s to form the plural.