13-year 这里的横线是将两个词连成了一个词,重点在后面的year,即可以把它当做一个形容词词看,一般我们说she is a 13-year-old girl要把13和year和old中间都加上横线,就是一个名词.因为是一个词,所以不需要加s做为复数.
13 years这里面就是两个独立的词,13修饰years,s在这里就是做复数用.
我们可以说一些很长的词,比如she is just the i-know-verything-but-i-will-not-tell-you-anything girl,这里连字符连的都算一个词,当然这句话你也可以说she is just the girl who knows everything but she will not tell you anything.
明白了吗?
