为什么羊的复数不加s：因为羊sheep是集体名词，没有复数形式，sheep作为名词，是可数名词，但单复数同形，及羊的单数是sheep，复数也是sheep，sheep还可引申为“胆小”的含义。
sheep的基本含义及用法介绍
sheep作为名词，意思为羊;绵羊。复数形式也是sheep。例句有：
1、The sheep had got out and were loose on the road.
那些羊跑出来在路上自由自在地走动。
2、The path was no better than a sheep track.
那条小路简直就像是羊肠小道。
3、At clipping time sheep need to be penned.
在剪羊毛时，需要把羊圈起来。
4、The minister preached a sermon on the parable of the lost sheep.
牧师讲道时用了亡羊的比喻。
5、I'd kinda like to have a sheep farm in New Mexico
我有点儿想在新墨西哥州开一个牧羊场。
6、Dogs were allowed to roam free and 48 sheep were killed.
没有对狗加以约束而让它们四处乱跑，结果有48只羊被咬死。
7、Sheep were munching their way through a yellow carpet of leaves.
绵羊边吃边走，穿行在厚厚的一层黄色落叶中间。
8、Their father was helping to dip the sheep.
他们的父亲在帮忙给羊群洗药浴。
9、Stefano used a motor cycle to herd the sheep
斯特凡诺骑一辆摩托车放羊。
10、He wasn't sure exactly how a fox could have got into the sheep's pen.
他不是十分确定狐狸是怎么进入羊圈的。
