1．在发音以辅音音素开头的单词前用a，在发音以元音音素开头的单词前用an。
a university，a useful animal，a European car，an hour，an honest boy，an Xray，an ugly man，an honor等。
Judging from his words，he must be an honest man.
从他的话判断，他一定是一位诚实的人。
As is known to us，Russia is a European country.
众所周知，俄罗斯是一个欧洲国家。
2．不定冠词a/an放在单数可数名词前，泛指一个人或一个事物。
They found an apartment in America where they lived only half a year.
他们在美国找到一个公寓，在那里待了仅仅半年。
3．表示泛指。说话人第一次提及某人或某物时通常用不定冠词。
It was a cold winter night and the moon was shining brightly.
那是一个寒冷冬天的夜晚，月亮在夜空中闪耀着。
