a 用於讀音以輔音開頭的單詞前；an 用於讀音以元音開頭的單詞前。
例如：
1、He had been spotted by an alert neighbour. 一個警覺的鄰居發現了他。
2、That clock's an hour fast. 那個鐘快了一個小時。
3、His speech was an exposition of the genesis of the conflict. 他的演講闡述了衝突的起源。
4、My friend needs abilingual dictionary.我的朋友需要一本雙語詞典。
5、There is a bird sitting in the tree.樹上臥著一隻鳥。
6、He has simply been exposed as an adulterer and a fool. 有人揭發他就是個奸夫和白痴。
7、Would you like asandwich?要來份三明治嗎？
8、There was a terrible sadness in her eyes 她眼神中流露出深深的悲傷。
9、I spend a lot on expensive jewelry and clothing.我花很多錢購買昂貴的首飾和衣服。
10、I had the beginnings of a headache.我有了頭痛的先兆
a 用於讀音以輔音開頭的單詞前；an 用於讀音以元音開頭的單詞前。
例如：
1、He had been spotted by an alert neighbour. 一個警覺的鄰居發現了他。
2、That clock's an hour fast. 那個鐘快了一個小時。
3、His speech was an exposition of the genesis of the conflict. 他的演講闡述了衝突的起源。
4、My friend needs abilingual dictionary.我的朋友需要一本雙語詞典。
5、There is a bird sitting in the tree.樹上臥著一隻鳥。
6、He has simply been exposed as an adulterer and a fool. 有人揭發他就是個奸夫和白痴。
7、Would you like asandwich?要來份三明治嗎？
8、There was a terrible sadness in her eyes 她眼神中流露出深深的悲傷。
9、I spend a lot on expensive jewelry and clothing.我花很多錢購買昂貴的首飾和衣服。
10、I had the beginnings of a headache.我有了頭痛的先兆