exciting前面介词用a 还是an

exciting前面介词用a 还是an exciting前面用定冠词a还是an exciting time to watch a football game前面填What an还是How an? 形容词是元音名词是辅音前面应该是a还是an比如说 exciting story 是 a exciting news还是 an exciting news? exciting前加a还是an onion前用介词a还是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 mp4前面用an 还是a egg前面用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