不定冠词a和an的用法口诀
(1)表示“一”这个数量，意思和one差不多，但数的概念没有one强烈。如：I want to buy a house. 我想买一套房子。He will be back in a day or two.他将在一至两天内返回。
这种用法在表示重量、长度、时间等的词组中特别普遍。例如：a pair of socks, a hundred, two yuan a metre, eight hours a day, twice a week, a cup of tea, in a word, one at a time.
(2)表示人或事物的某一类 ，区别于其他种类。如：I am a boy. 我是男孩。Pass me a pen, please.请递给我一支钢笔。A plane is a machine that can fly. 飞机是会飞的机器。A teacher must love his students. 老师必须要爱他的学生。
(3) 表示某一类人或事物中的任何一个。如：This is an apple. 这是一个苹果。His father is a teacher. 他爸爸是一名教师。 She picked up a magazine and began to read. 她拿起一本杂志，开始读起来。
(4)用于泛指人或者事。如：A boy is waiting for you. 有一个小男孩在等你。He lent a story-book to me.他借给我一本故事书。Before she left for London, Sally sent me a picture of herself. 萨莉动身去伦敦前送给我一张她的照片。 A grammar book is necessary to a language learner. 对语言学习者来说，语法书是很必要的。
(5)用于某些词组中。如：have a rest（休息）, have a look（瞧瞧）, have a good time（过得开心）, take a shower（淋浴）, take a break（休息）, take a bath（洗澡）, get a cold（患感冒）, get a fever（发烧）, make a living（谋生）, make a difference between（区分）, in a moment（过一会儿）, in a sense（在某种意义上）, in a way（在某种程度上）, as a result（因此）, as a rule（通常）, as a whole（总体上说）。
注意：a用在辅音之前，如：a boy, a useful invention等；an用在元音之前，如：an hour, an old man等。如：The horse is a useful animal. It takes half an hour to cook.
