Is there a simple rule for "the" article?

Valerie has given a good simple rule.

There are other details that can help you when that rule is unclear.

-/the - the zero article and the definite article

There are many rules for when to use the definite article, 'the'.  In the following, I have written only the most general and functional rules.  Specific rules for certain nouns are not included.

Use the definite article when:

1. a noun is defined by a phrase that follows it.

The car that we used last night. (Which car? The one we used last night.)

2. it is obvious from the context what is referred to. (Example needed.)

3. we mention the noun a second time or use a substitute noun.

There is a cat on the gate.  The feline jumped down after a mouse. (Which feline? The cat which was on the gate.)

4. we refer to something unique (including superlatives). 

This is the best coffee I've ever had! (Which coffee is it? The best.)

The President of the U.S.A. is Barack Obama. (a president is a unique person.)

Use the zero article/ don't use the definite article:

1. when we focus on the type of institution rather than a particular or specific building.

I went to university in Canada.

2. before plural nouns that are general, not specific.

I love cookies! BUT:  I love the cookies you gave me!

3. before abstract nouns.

We have hope and preserverance.

The definite article is used before singular and plural nouns when the noun is specific or particular. The signals that the noun is definite, that it refers to a particular member of a group. For example:

"The dog that bit me ran away." Here, we're talking about a specific dog, the dog that bit me.

"The" is a definite article, it is used as followings:

1. the+unique nouns

the earth

the stars

the sun

2. with nouns mentioned for the second time.

I have a car, the car is small but enough for me.

3. with modified nouns

the boy in black is my brother.

4. the+ a noun by the reason of locality can be represented a particular thing

the car in the garage.

5. the+ singular noun that represents the whole class of things

the lion is dangerouse.

6. the+adj.

the rich should help the poor.

7. with names of rivers, seas, groups od islands, chain of mountains, and plural names of counties

the dead sea

the hawaii

the united states

the united kingdom

8. with names of people which has a title

the simpsons

the adams

And zero article with proper nouns, abstract nouns, meals, names of games, parts of body or clothings, and locatives.

Kojin is a student.

Death is a real fact.

I have my breakfast at 8 am.

I like football.

Raise your hand.

I go home.

I will give an example needed in the definite article number 2 of which Kimberly had explained : Open the door. Is it right?

Part 1 Known because we already talked about it
The listener might know which one we mean because we’ve already talked about the thing in our conversation (or piece of writing).

• I bought an apple and an orange. The apple was delicious. We use ‘an apple’ and ‘an orange’ at first because we think the person listening won’t know which apple or which orange we are talking about But the second (or third or fourth…) time we talk about something, we can use ‘the ’because the listener knows which one. He or she knows because we’ve already said which one – it’s the apple that I bought yesterday and not another apple.

A female student came in. The girl… (‘The girl’ is the same person as ‘a female student’ so we have already introduced her).

Part 3: Known because we mentioned something connected

We can also use ‘the’ to talk about things which we haven’t talked about directly in
the previous conversation but which we can understand from something else we’ve
said. In the example below, we know that houses usually have doors:

• We arrived at a house. The door was open. (In this case we know ‘the door’ is the door of the house we'’ve just talked about. We think the listener will understand which door we mean, but we use ‘a’ when we  introduce the house because we don’t think the listener will know
which house we mean).

• I wanted to buy a new coat but the price was too high. (When you buy something, there’s usually a price, so we think the listener will understand that we mean the price of the coat).

Part 3: The listener / reader might know which one
we mean because it’s obvious from the physical situation  near us Sometimes the listener knows which one we mean because of the place we are in. We might be able to see the thing we are talking about. For example:

• Pass me the glass (if there’s only one glass we can see, then the listener knows
which one, because there’s no other choice).

On the other hand, if there is more than one, the listener probably doesn’t know which
one I mean, so I have to use ‘a / an’:
• Please close the window (only one is open).
• Please close a window (three are open: I want you to close any one of them).
If there is more than one and we want them all, we can still use ‘the’:
• Close the windows (there are three windows open, but I want you to close all
of them).

The situation we are in could also be something bigger, like the town, city or country
we are in. For example:
• You can buy apples in the supermarket (the supermarket in our town).
• The library is on Main Street (the library in our town).

Part 4: It’s obvious which one we mean because we say so in the sentence
Sometimes the listener knows which one because we make it clear in our sentence, by
using certain grammar or vocabulary.

1: Relative Clauses
We can use different grammatical structures to say which one we mean. One which
we often use is a relative clause. For example:
• John knows the girl who I met yesterday Remember, it needs to be clear which one we mean from the relative clause. If it’s not obvious, and the listener doesn’t know which one we’re talking about, we can still use ‘a’ even if we use a relative clause. For example:

• I saw John talking to a girl who I met last night. (Even though there is a relative clause, we still don’t know exactly which girl – I met several girls last night, so we can use ‘a’).
• I saw John talking to the girl who I met last night. (I met only one girl last night)

Prepositional Phrases
We often use prepositional phrases (on the table, by the station) to say which one we
mean. For example:
• Pass me the book on the table (there’s only one book on the table, so it’s clear
which one we mean).
• I often go to the supermarket by the station.
Be careful – we only use ‘the’ with a prepositional phrase if it tells us which one.

‘The back of’ etc
Another grammatical structure that tells us which one is ‘of’ phrases that talk about a
certain part of something. Because something only has one back, for example, we use
‘the’ when we talk about it, as the listener must know which one we mean. For example;

the back of         I wrote her number on the back of my notebook.
the front of        The front of the dress was blue.
the middle of        She sat down in the middle of the floor.
the top of              He stood at the top of the stairs and waited.
the bottom of       The answers are at the bottom of the page.
the edge of            I tripped on the edge of the pavement.
the beginning of    At the beginning of the book, she is living in Paris.
the end of              They got married at the end of the film.
the height of         What’s the height of Mount Fuji?
the length of        He ran the length of the football pitch.
the size of            I was amazed at the size of his house.
the weight of       I can’t guess the weight of the cake.
the title of           What’s the title of the film we saw last week?
the price of          The price of flats here is very high.

Superlatives
Another grammatical structure that tells us which one is a superlative.
If we use a superlative (the tallest student in the class) then there is obviously only one (or one group) of the thing we are talking about. There is one student who is the tallest in the class, and because it’s clear which one we mean, we can use ‘the’:

• She’s the most beautiful girl I’ve ever seen.
• It’s the best café in London.
• John and Lisa are the most intelligent students here.
• This bowl is the biggest.

Certain Adjectives
There are some adjectives which are often used to talk about only one thing (or one
group of things). For example, if I say ‘the first bus’ I use ‘the’ because only one bus
can be first, so the listener knows which one I mean.
Here’s a list of some adjectives we often use with ‘the’:
same     He was wearing the same t-shirt as me.
next       Let’s get on the next train that comes.
last         We caught the last bus home.
first / second /third …

She bought the first dress she found.
I’d like the second cake on the shelf.
only      Coffee is the only thing I want now.
right     It’s the right answer.
wrong    * I’m afraid that’s the wrong bottle.
usual      We went to the usual restaurant.

*I know it is a bit strange to talk about the wrong answer when there are usually lots
of wrong answers to any question, but we do! (We don’t use ‘the’ when we are using these words alone to order ideas in a sentence. First, you mix the flour and the water. Next, …)
 ‘Next’ and ‘last’ with time expressions:

However, be careful about ‘next’ and ‘last’ when they are used with time expressions
– in this case they usually don’t take ‘the’. When ‘next week’ means ‘the week after
this one’ we don’t need ‘the’:

• I’m going to visit my brother next week (NOT: the next week)
When ‘last week’ means ‘the week before this one’, we don’t need ‘the’:

• I saw David last week (NOT: the last week)
This is the same for similar expressions like: ‘next month’, ‘next year’, ‘last month’,
‘last year’, ‘last night’, ‘last summer’, ‘next winter’. It’s also true with days, like ‘next
Tuesday’, ‘last Sunday’)
• I saw the new film last month (not: the last month)
• We’re going on holiday next month (not: the next month)
• She will graduate next year (not: the next year)
• I went on holiday to Bolivia last summer (not: the last summer)
(‘Last’ has two meanings. It can mean, ‘coming at the end’ (opposite of first) – in this
case, we need ‘the’, as only one thing (or group of things) can come at the end.
Second, ‘last’ can mean ‘previous / the one before today’ like last week, last month.
In this case we use ‘no article’)

Unique things
In the section about the physical situation, we saw that we can use ‘the’ when the
listener knows which one we mean because of the room or building or town we are in.
If there is only one of something in the room, for example, we can use ‘the’ because
it’s clear which one we mean. For the same reason, we can use ‘the’ with nouns when
there is only one in the country we are in.
• The Queen was on television yesterday (if I am in England when I say this,
you will understand that I mean the Queen of England).
• I think we should support the government (I mean the government in our
country).
Sometimes, there is only one of something on Earth, or even in the universe. We use
‘the’ with these words, as it’s clear which one we mean. For example, we use ‘the’
with ‘sun’ as there’s only one sun (close to us, anyway!) and everybody knows which
one we mean.
• The sun was very hot that day.
In the same way, we often use ‘the’ if there is only one group of something:
• I loved learning about the planets in school.
Here is a list of some words that are often used like this:
1. the sun Don’t look directly at the sun.
2. the moon She could see the moon from her bedroom window.
3. the stars The stars are difficult to see clearly.
4. the sky The sky was a lovely shade of blue.
5. the universe The universe is vast.
6. the planets I would love to visit the planets.
7. the world It’s the best city in the world.
8. the solar system The alien travelled to the edge of the solar system.
9. the earth The earth looks small from space.
10. the equator The equator goes through Ecuador.
11. the north pole The north pole is surrounded by snow.
12. the past It must have been difficult to live without electricity in
the past.
13. the future Can you imagine what life will be like in the future?
14. the present There’s no time like the present.
15. the internet I looked the word up on the internet.
16. the environment We need to think about protecting the environment.

We also use ‘the’ with time periods (like ‘the twentieth century’) because there is only
one of them. We say ‘the sixties’, ‘the eighteenth century’, ‘the Middle Ages’:
• They were the most popular pop group in the sixties.
• What was it like to live in the Middle Ages?
• The French revolution was in the eighteenth century.
We also usually use ‘the’ with grammatical terms such as ‘the definite article’, ‘the
past simple tense’ and so on – we imagine that there is only one of these things (at
least in the language we are talking about).
• Put these sentences into the present tense.
• You need to use the definite article if the listener knows which one you mean.