how do we normally make the plural form of a singular nounthat endi in f or fe~?

Answer 1

Do you know what the plural of leaf or knife is?

Answer 2

The rule is to drop the -f or -fe and replace it with -ves.
leaf > leaves
knife > knives

For other plural forms, you can look them up in a dictionary. It will give you the plural form of irregular nouns.