以f或fe 结尾的名词变复数时：
a.加s,如：belief---beliefs roof---roofs
safe---safes gulf---gulfs；
b.去f,fe 加ves,如：half---halves
knife---knives leaf---leaves wolf---wolves
wife---wives life---lives thief---thieves；
c.均可,如：handkerchief:
handkerchiefs / handkerchieves