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.