通常以下情况用the
a)单一的事物，也就是只有一样、一件或一套
例：the world, the sky, the atmosphere, the sun, the ground, the climate, the horizon, the human race, the environment, the travel industry, the Internet
b)宽广的地理区域
例：the beach, the town, the sea(side), the land, the country(side)
c)时间范围
例：the past, the present, the future
需注意以上部分涉及到广义的名词前可以什么都不加，例如
Climate is one of the many factors involved in changing farming methods. （或The climate...）
These flowers grow best in sandy soil and sun.
In autumn the temperature difference between land and sea decreases. （或...the land and the sea...）
以上名词如果是特指或特例，可以用a/an。例如：
I could see the plane high up in the sky.（天空就是一个天空，非特指，用the sky）
When I woke up there was a bright blue sky. （特指我醒来时的天空，用a sky）
What are your plans for the future? （泛指未来这个时间概念，用the future）
She dreamt of a future where she could spend more time painting. （特指她脑海中想象的未来，用a future）
谈论某一类事物的共性时，用the+可数名词单数形式，例：
The computer has revolutionised publishing. （这里说的是电脑这种物件整体带来的效应，而非谈论个体，所以不能用a computer）
The computer is an important research tool. （理由同上）
A computer is an important research tool. （这句话既可以用来指所有的电脑，也可以特指某台电脑，所以可以用a computer）
谈论一类物品时，可以用可数名词复数替代the+可数名词单数，例：
Computers are an important research tool.
谈论一个或一组特定物品时，可以用the+可数名词复数或不可数名词，例：
The computers have arrived. Where shall I put them?
The music was wonderful. I could have listened to the orchestra all night.
当定义某物或强调一群人或物中的特定种类时，通常用a/an而不是the，例：
A corkscrew is a gadget for getting corks out of bottles.
A garden is there to give you pleasure, not to be a constant worry.
有些名词既可以作不可数也可以作可数，作为整体或观念时为不可数，作为一个或几个个体示例时为可数。当这些名词是可数名词时，可以用a/an或复数形式。例如：
I don't drink coffee. （咖啡泛指时是不可数名词）
Would you like a coffee? （特指a cup of coffee）
He shook with fear. （恐惧泛指是不可数名词）
He has a fear of heights. （针对特定情况的恐惧用a）
此类名词还有conversation, grammar, importance, iron, pleasure, shampoo, sound. 其中有些名词像grammar, iron作可数和不可数时意义不同。
讨论某人的工作或职业时用a/an，例：
She was a company director when she retired.
Against her parents' wishes, she wants to be a journalist.
然而谈论某人的职位，且该职位通常只有1个人担任时，用the或不加冠词，例：
She's been appointed (the) head of the company.
I'm a production manager at Fino. （可能这家公司有好几个生产经理，所以用a）
the position of, the post of, the role of+职位时，不加任何冠词，例如：
Dr Simons has taken on the position of Head of Department.
通常人名或地名前不加冠词，但是以下情况除外
a)为了区分重名的人，例：
That's not the Stephen Fraser I went to school with.
There was a Stephen Fraser in my class. （班里有个人名叫Stephen）
b)强调该人为名人时用the，例如
Do they mean the Neil Armstrong, or someone else?
c)人名前有形容词或职业名词，例如
the late Michael Jackson, (the) artist Joseph Turner
d)提到一家人时用the，例如
The Robinsons are away this weekend.
如果某人很优秀，提到该人的人名时加a/an或不加冠词，例如：
Majid plays tennis well, but he'll never be (a) Roger Federer.
提到某特定生产厂商制造的单件产品或某特定艺术家的作品加a/an，例如
I've jsut bought a Mercedes.
Do you think it could be a Van Gogh/a Rembrandt?
提到不认识的人可以在人名前加a/an，例如：
Dr Lee is here for you. （听话人认识Dr Lee）
There's a Dr Amy Lee on the phone. （不认识）Do you want to talk to her?
讲故事或笑话时，通常用this而不是a/an来带出一个新人物/事物。this可以强调即将成为话题点的人或物，例如：
As I was walking along, this spider(=a spider) landed on my head, and...
This man (=a man) goes into a chemist and he says...
名词或定义短语前的形容词最高级前加the，例如
He is the finest young player around at the moment.
但是口语中，如果形容词最高级后没有带名词或定义短语，则the可以省略，例如：
A：Why did you decide to stay in this hotel?
B：It was (the) cheapest. 比较 It was the cheapest I could find.（cheapest后面带定义短语，不能省略前面的the）
复数或不可数名次泛指时前面不加冠词，但假设听话人或读者知道谈到的人或物，或有单词能明确所说的人或物时，前面加the，例如：
The government has promised not to tax books. （书是泛指）
The books have arrived. （特指定的书）
Music played an important part in his life. （泛指音乐）
I thought the music used in the film was the best part. （特指电影中的音乐）
泛指节假日、一年中的特定时刻，月、一周中跟Easter, Ramadan, New Year's Day有关的日期前不加冠词，例如：
I'll see you on Saturday. （=下周六）
We met on Saturday. （=上周六）
They came on a Saturday as far as I can remember. （只需要知道他们是周六来，至于是哪个周六不重要）
They came on the Saturday after our party. （特指聚会后的那个周六）
泛指春夏秋冬和New Year（包括整个新年放假时间段）时，前面不加冠词或加the，例如
In (the) summer I try to spend as much time as I can in the garden.
特定的春夏秋冬前，加the，例如
I'd like to go skiing in the autumn. （=今年）
I first went skiing in the spring of 2002.
I'll see you again in the New Year. （即将到来的明年的新年）
描述一个特殊的节假日、季节或某个特殊时刻，并且将其与其他日子比较来衬托其特殊性时，可以用It/That was ... +a/an+名词+修饰语，例如：
That was a winter I'll never forget. （与其它冬天比有其特殊性）
That was the winter we went to Norway. （只是陈述一个特定冬季）
midnight, midday和noon前不加冠词，例如：
If possible, I'd like it finished by midday.
Midnight couldn't come quickly enough.
dawn前可以加或不加the，例如
He got back into bed and waited for (the) dawn.
听说双方都知道的某一天的早中晚前可加the，例如：
I enjoyed the morning, but in the afternoon the course was boring.
Morning is the time I work best. （泛指早晨不加冠词，此处用The morning...也可以）
I'll be there by (the) morning/evening. （但是只能...by the afternoon，不能...by afternoon）
I waited all morning. （比all the morning/afternoon更常用）
'You look upset.' 'Yes, I've had a terrible mroning.' （与其它早晨相比）
搭乘交通工具用by+交通工具，例如：
I generally go by bus to work. 也可以说成I generally take the bus to work.