因为这是语言习惯.这个没有办法通过逻辑或者原理的方式来解释.所以更多的只能靠记忆和多加使用来巩固.
下面帮楼主总结了一些以f或fe结尾的词语的复数变化,希望能帮到你.
以f或fe结尾的词,多数直接加-s:
chief chiefs reef reefs
cliff cliffs cuff cuffs
roof roofs fife fifes
belief beliefs safe safes
gulf gulfs strife strifes
但有几个名词,需变f为v,再加es:
elf elves leaf leaves
calf calves thlef thleves
shelf shelves loaf loaves
half halves life lives
self selves wife wives
wolf wolves knife knives
有个别名词,可加-s或-ves:
hoof hoofs,hooves wharf wharfs,wharves
scarf scarfs,scarves dwarf dwarfs,dwarves
beef beefs,beeves staff staffs,staves