half a sandwich,half hour,one and a half hour.这些half的用法都不相同,该怎么理解?half hour正确吗，应该用half an hour?

half a sandwich,half hour,one and a half hour.这些half的用法都不相同,该怎么理解?half hour正确吗，应该用half an hour? half a a half half a a half half Half half a half apples还是a half apple a half与half a的区别 a half mile 还是half a mile half a day 为何不说half day.类比half apple 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 a half million