The Basics  

English puts "a" and "the" before a noun. (As you read this, notice how many times "a" and "the" come before nouns.) It is important to know why English speakers use words like "the" and "a". English speakers like to know which thing you are talking about. We like that detail. But only about 25% of languages in the world use words like "a" and "the", so it might be difficult for you to understand why we use these words. Many times, the situation makes it clear which thing you are talking about. If you are holding a pen and you say, "This is pen", maybe I can understand. But sometimes adding "the" or "a" gives us a little more control over saying which pen it is. It's a nice detail for English speakers. It could be my pen, your pen or the pen I got at Disneyland. So, words like "a", "the", "my", "your", "this" and "that" say which thing you mean.

Think about this situation. You are talking to your friend (a native speaker) and you say this:

You: Where is key?
Friend: Which key?
You: My key.

If you do not include some word to show which one, native speakers might be confused, or need to ask you to be clear: If you say nothing, a native speaker will ask, "Which one? Are you looking for the car key, your key or the key to the house?" You can answer the question before it is asked. Use "a" or "the".

"A" and "the" show when a thing is known to both the speaker and listener, or not.

• The shows that both speaker and listener know the thing being talked about. "The" is a lot like "that". When you use "the" you are kind of pointing at a thing and saying, "That one →!"
• A/an can show three things:
  (1) That the thing is not known to either the speaker or the listener.
  (2) That any one is OK. Which one is not important.
  (3) That both the speaker and the listener don't know which thing.
  "A" doesn't point to any one special thing.

The key question to ask yourself is:

Do you know the thing I mean?

If the answer is "yes":

• use the.

If the answer is "no" or "it's not important":

• use a/an (if the noun is singular countable)
• or nothing (if the noun is plural or uncountable).

Look at the picture below ↓

1. First, you need to know if the thing you are talking about is countable or uncountable.
2. Then, you need to choose THAT, ANY or ALL.

A and THE: Image: Poligo

Here are some more examples:

Note:

1. Articles can never be combined with other determiners. For example, we cannot say 'the my cat' or 'a this car'.
2. It is important that you understand countable and uncountable nouns to use "a" and "the" well. 

The

"The" is a definite article and shows that the speaker and listener know 'which one/s' they are talking about. It is clear (definite) which one they mean. "The" points like "that" to something special. There are three main ways for the noun to be known. Look at the table below:

Often the situation makes it clear which thing you are talking about. Look at the image below:

You can see that there is only one red circle on the right. If I say "the red one", you know which one it must be. 

A

"A" and "an" are indefinite. We use them when it is not clear (indefinite) to either the listener or the speaker 'which one' is being talked about. If only the speaker knows 'which one', she should use "a". Because "a" means "one", it can only be used with singular countable nouns.

In other words, if both of these things are true:

>> Learn more about countable nouns

Choose A or AN by sound, not spelling!

"A" is used before words that start with a consonant sound: 

A cat

A yellow banana

A university (you-ni-ver-si-tee)

"An" is used before words that start with a vowel sound:

An umbrella

An oyster

An intelligent person

An S (ess)

The table below shows cases when to use "a" or "an":

No Article  

If you can't use the and you cannot use a, then you don't need an article. For example:

I like oranges (any oranges, plural)

Give me water (any water, uncountable)

>> Learn more about exceptions to A and THE