以f结尾的单词什么情况下是变为ves什么情况下是直接加s

问题描述:

以f结尾的单词什么情况下是变为ves什么情况下是直接加s
如leaf是变为leaves还是变为leafs

leaf-leaves(树叶)scarf-scarves/scarfs(围巾)handkerchief-handkerchieves/handkerchiefs(手帕)knife-knives(刀子)thief-thieves(小偷)life-lives(生命)self-selves(自己,myself-ourselves)...