There are some basic rules for pluralising nouns, such as changing y to ies and fe to ves.
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.
There are words that don't change at all in the plural.
There are also words that change completely in the plural.
Sometimes we just have to learn the plurals, although a dictionary can be very helpful.