其他应注意的数的表达形式
1)a three-year-old boy 比较The boy is three years old
2) five thousand people / several thousand people比较 thousands of people
表达具体数字时，thousand不加 "s"，后面不加 "of "； ten， million， billion情况以此类推。
3) One-third of the students have passed College English Test Band IV.
Two-thirds of the students have passed College English Test Band IV.
One-third of the population in this area is living below the poverty line.
（这个地区有1/3人口生活在贫困线以下。)
注意：几分之几的表达形式；动词的单复数由后面的名词数决定。
4) a looker-on 比较two lookers-on（旁观者) / a brother-in-law比较two brothers-in-law（姻亲兄弟)
5) my father， together with （along with / with / as well as my mother and brothers) 结构中，主语作单数处理，即my father
