Study the rules:

• Use ending -es if a noun ends in -o.

Examples: potato - potatoes, tomato - tomatoes.

But: add only -s to words of foreign origin or abbreviated words ending in -o.

Examples: piano - pianos, photo - photos

• If a noun ends in -y following a consonant, change the -y to -i and add -es.
  

Examples: baby - babies, story - stories.

But: if -y follows a vowel, just add -s.

Examples: day - days, boy - boys.

• Some nouns that end in -f or -fe change the -f or -fe to -v and add -es.
  

Examples: knife - knives, wolf - wolves

But: some other nouns ending in -f or -fe just add -s.

Remember: handkerchief - handkerchiefs, safe - safes, cliff - cliffs.

• Names of some creatures do not change their forms in plural.

Remember plural forms of the following words:

fish - fish

deer - deer

sheep - sheep

Study more rules and do more exercises: Basic English Grammar Plural Nouns - 1