excellent 前面用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 exciting前面用定冠词a还是an use前面用an还是a? engineer 前面用A 还是AN egg前面用a,还是an? uncle前面用a,还是an 字母u前面用a还是an American前面用a还是an white前面用a还是an