two thousand为什么不加S而two months加S

two thousand为什么不加S而two months加S two thousand of为什么thousand不加s two thousand中thousand加s吗? 为什么two-year-old的year不加S而two-hundred-words的word要加S 为什么two pieces of cake 中的cake不加s? two shoe stores为什么shoe不加s 为什么 two hundred hundred不用加s? 为什么every student student不加s,every two hours 加s million thousand 的后面为什么不加S two meters long中的meter为什么直接加s 而two days' holiday 中的day为什么变所有 two,three,four,five,six,seven,eight,nine thousand后面要不要加s three hundred pounds 为什么加S four thousand people 为什么不加S