半小时用英语讲为什么是half an hour而不是a half hour

半小时用英语讲为什么是half an hour而不是a half hour half an hour or a half hour? a half hour还是half an hour? 是half hour还是half an hour? a half hour ,half an hour都是"半小时的意思吗?　　　"一个半小时"是什么? 一个半小时 英语：是one and a half hour 还是 one and a half hours half an hour‘s walk为什么不能讲成half-an-hour walk? half an hour中的half词性为什么是名词? a half of an hour an hour and half还是an hour and a half half an hour 和a half hour有什么区别? a half hour later\after half an hour\after half an hour late