为什么two-year-old的year不加S而two-hundred-words的word要加S

为什么two-year-old的year不加S而two-hundred-words的word要加S two hundred old people 的people要不要加s 为什么 two hundred hundred不用加s? 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 two hundred?要不要加s? two thousand为什么不加S而two months加S 为什么Six Years Old.中Year要加S呢? he is 18 years old.为什么year要加S an eight-year-old boy为什么year不加s 这属于什么语法 他是一个十岁的男孩怎么翻译?years old ,year old 在什么时候不加s? two year's time前加什么介词? 七年之痒的翻译：the seven year itch中的year为什么不加S