要用a还是an?

要用a还是an? a还是an uniform 用a还是an 是a还是an an NBA还是a NBA an mp5还是a mp4 useful前是加a还是an a Australian还是 an Australian an eraser 还是 a eraser a还是an important person a invitation还是an invitation an UFO还是 a UFO a egg还是an egg a European还是an European uniform前+a还是an a aunt 还是an aunt a man还是an man? a object 还是 an object