f结尾的单词变复数怎么变

When the ox falls, they sharpen their knives.

牛倒了，他们就磨刀。

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

在过去，刀和布匹可以被当作钱用。

In some countries, people eat with knives and forks.

在一些国家，人们用刀叉吃饭。

Kanzi can also use knives and play easy computer games.

坎兹还可以使用刀具、玩简单的.电脑游戏。

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

同样的原理可以用于使小刀更锋利或者清扫城市大街时。