学语法遇到两难题“以o结尾的加s或es"“以f或fe结尾的词去掉f或fe加ves或直接加s”我想问怎样加s怎样才加es 怎样加ves或s

Common noun ending in -o + es are as follows:
Negroes and heroes like tomatoes and potatoes.
That is, the other common nouns ending in -o are then all followed by only -s.
Common nouns ending in -f or fe are made plural by adding -s directly to the ending;
roofs, beliefs. chiefs.
Either handkerchiefs or handkerchieves.
The others are made plural by changing -f,or -fe to - ves.