1. 不定冠词的用法

(1) a用在子音前; an用在母音前。
An hour 

(2) 表同种类的全体, 此时a带有any的意味。
A dog is a faithful animal. (=Dogs are faithful animals.) 

(3) 等于one。
I have a sister and two brothers. 

(4) 等于the same.
Birds of a feather flock together. 

(5) 等于per“每一......”。
We work eight hours a day. Ninety miles an hour.

(6) a = a certain, 表说话者不认识此人。
Do you know a Mr. Smith? 

(7) 物质名词或抽象名词加a (an)即成普通名词，
Asparagus is a grass. (a = a kind of).
This is a good cloth for summer. 

(8) “have (or take) + a + 抽象名词”，与抽象名词的动词形同义。
Let us take am. = Let usm. 

2. 定冠词的用法
 
(1) 由于上下文的关系, 某名词所指的东西已非常明显, 或已有一定的范围, 该名词应加the。
Shut the door.
Please pass me the caster. 

(2) 表该名词的总称。
The cow is a useful animal. (= Cows are useful animals.) 

(3) 前面已提过的名词，再度提到时前面加the。
Once upon a time there was a little boy who lived in a cottage.
The cottage was in the country, and the boy lived there all his life. 

(4) 用于为片语或子句所修饰的名词之前。
This is the book that I promised to lend you. 

(5) 用在宇宙（独一无二）的天体名词之前。
The sun; the moon; the sky; the earth.

(6) 用在方位，方向等名词之前。
The sun rises in the east and set in the west. 

(7) 乐器的名称前要加the。
To play the piano (violin, guitar, flute¹, etc.) 

(8) 表示单位的名词前要加the。
Gasoline is sold by the gallon.
Meat is sold by the catty. 

(9) 在最高级的形容词或序数(the first...)之前要加the。
Taiwan is the best place to live in. 

(10)专有名词在下列情况可加the。
(A) 海、河、运河的名称：The Thames; the Bahamas;
(B) 复数形的专有名词: The West Indies; the Bahamas;
(C) 船泊的名称: The Queen Mary; the Flying Scotsman; the Comet;
(D) 书报、杂志、经典的名称： the Times; the Central Daily News; the Koran;
(E) 公共建筑、机关的名称: The Ritz Hotel; the Ministry² of Education;
(F) 国民的总称: the French; the Japanese; the Chinese;
(G) 用在姓氏的复数前指其全家人: The Chens will move to the country.