为什么 a four-year-old boy中的year可以不加复数,而分数two-thirds中连字符后的third

为什么 a four-year-old boy中的year可以不加复数,而分数two-thirds中连字符后的third 英语用法之分数连字符何时用?书中a third,two-thirds,three quarters；a quarter, two third 2/3如果加了连字符是不是two—thirds?加了连字符third为什么加了s?一般连字符表示数字 为什么two-year-old的year不加S而two-hundred-words的word要加S 英语分数表示方法三分之二,two thirds一定是对的,但是我要是加连字符呢?是two-thirds还是two-thi a four-year-old boy可以说成a four-year old boy 为什么要说the/a two-year-old boy 不能说two years old boy 所有格的两种形式,1连字符形式 ten-year-old 2 ten year’s old 可以 英语中连字符的用法,如79-year-old 79 years old an eight-year-old boy为什么year不加s 这属于什么语法 请问:是a four years old boy?还是a four year old boy?正确 英语小问题三分之二用英语怎么说?中间有连字符的那种.two-third还是two-thirds?可是有连字符不是就变成形