英语中定冠词a/an与the的区别?

an, a是不定冠词,仅用在单数可数名词前面,表示“一”的意义,但不强调数目观念.a用在以辅音（指辅音音素）开头的词前, an用在以元音（指元素音素）开头的词前,如：
a history class an island
a university an elephant
a hero an old man
不定冠词的用法：
1. 表示人或事物的某一类
A plane is a machine that can fly.
2. 表示某一类人或事物中的任何一个.
This is an apple.
His father is a teacher.
3. 泛指某人或某物,但不具体说明何人何物.
A comrade is waiting for you downstairs.
I met an old man on my way to school.
4. 表示“一个”的意思
He will be back in a day or two.
He tried to send a picture of a face.
5. 有时也表示“每”的意思
We have three meals a day.
我们一日三餐.
He walks ten miles an hour.
他一小时走10英里.
We have six classes a day.
我们一天上六节课.
He drives the car at thirty miles an hour.
如果把不定冠词后的名词变为复数,只把不定冠词去掉还不行,还得补上一个数词或不定代词（some, any）
The plan will be ready in a few days.
这是因为“一个”或“某个”变为复数时,就成为“几个”或“某几个”的缘故.
定冠词the具有确定的意思,用以特指人或事物,表示名词所指的人或事物是同类中的特定的一个,以别于同类中其他的人或事物,相当于汉语中的“那个”或“这个”的意思.它可以和单、复数名词,也可以和不可数的名词连用.II定冠词的基本用法：
1. 特指某（些）人或某（些）事物.
Give me the book.
Did you hear the talk given by Mr Li?
How do you like the film.
Have you got the letter?
2. 指谈话双方都知道的人或事物.
Where is the teacher?
Open the door.
3. 复述上文提过的人或事物.
I have a beautiful wallet, but the wallet was stolen yesterday.
4. 世界上独一无二的事物等（月亮、地球、天空、宇宙）
The sun rises in the east.
The earth goes round the sun.
the globe, the universe.
the atmosphere大气层
5. 用在方位名词前
in the south, in the west ,in the north
6. 用在序数词或形容词*前
The first thing I want to say is to listen carefully in class.
He is the tallest one in our class.
7、表示两者间“较……的一个”时用定冠词
The older of the two noblemen took a light.
He is the taller of the two boys.
8. 用在单数可数可数名词前表示一类人或事物.
The horse is a useful animal.
9. 定冠词用在形容词前,表示一类人或东西.
The rich, the poor ,the wounded ,the deaf