原则上说，a用于辅音前；an用于元音前。
1. 辅音前用a的例子：
He is a bit of a coward.他多少有几分怯懦。
The bread was as hard as a brick.这面包硬得像块砖。
I got a friendly reception from them.我受到他们友好的接待。
The only green thing was a lone pine tree. 绿色的东西只有孤零零的一棵松树。
A good driver varies the speed of his car according to the condition of roads.优秀的司机会根据路况不同而改变车子的速度。
I want to buy a hat and a pair of boots, among other things.除别的东西之外，我还想买顶帽子和一双皮靴。
His parents give him so much money that he’s got no motivation to get a job. 父母给了他很多钱，因此他根本就没有找工作的动力。
2. 元音前用an的例子：
But everything has an opposite.但一切都有相反的一面。
I have an American passport.我有一份美国护照。
It was an old pine tree in the backyard.这是后院里的一棵老松树。
He received an unfair treatment. 他受到了不公平的待遇。
About this time an incident happened. 差不多就在这个时候，发生了一件事。
After an absence of seven years I went home. 外出7年后，我返回家乡。
Please write an abstract of this scientific article. 请写一份这篇科学论文的摘要。
I didn’t receive an acknowledgement of my application. 对我的申请，我还没收到回信。