There are some basic rules for pluralising nouns, such as changing y to ies and fe to ves.
puppy puppies
knife knives
However, not all nouns follow the rules.
The plural of roof is roofs, not rooves, but the plural of hoof can be either hoofs or hooves.
Some nouns ending in o add es in the plural while others just add s.
tomato tomatoes
radio radios
There are words that don't change at all in the plural.
sheep sheep
fish fish
There are also words that change completely in the plural.
person people
mouse mice
Sometimes we just have to learn the plurals, although a dictionary can be very helpful.