outstanding 前面用a还是an

outstanding 前面用a还是an “outstanding”前面是用该a还是an? 是he was a once outstanding man还是he was an once ...这里应该用a还是用an 字母A前面用a还是an s前面用a还是用an university前面用“a”还是用“an”? year前面用a 还是用an exciting前面介词用a 还是an mp4前面用an 还是a egg前面用a,还是an? exciting前面用定冠词a还是an engineer 前面用A 还是AN 字母“n”前面用a还是an? ufo前面用a还是an useful前面用an还是a?为什么? 字母u前面用a还是an UFO前面用a an 还是the octopus前面用an 还是a