f結尾的單詞變復數怎么變

　　When the ox falls, they sharpen their knives.

　　牛倒了，他們就磨刀。

　　In the past, cloth and knives can be used as money.

　　在過去，刀和布匹可以被當作錢用。

　　In some countries, people eat with knives and forks.

　　在一些國家，人們用刀叉吃飯。

　　Kanzi can also use knives and play easy computer games.

　　坎茲還可以使用刀具、玩簡單的電腦游戲。

　　The same principle can be used to sharpen knives or sweep city streets.

　　同樣的原理可以用于使小刀更鋒利或者清掃城市大街時。