half a day=a half day,那为什么half a dozen 却没有a half dozen 这种说法?

half a day 为何不说half day.类比half apple half a day=a half day,那为什么half a dozen 却没有a half dozen 这种说法? half a day 为何不说a half day half a a half half a a half he often___(relax) half a day a half apples还是a half apple a half与half a的区别 a half mile 还是half a mile For a day and a half last spring 翻译... they spent half a day planting trees的同义句?they spent half a day planting treesit ___them half a day ___ ___the trees? a half 与half of 的区别? a half hour还是half an hour? half an hour or a half hour? Half On A Baby half a kilo of