1.Indefinite articles are used for previously unknown nouns that are being introduced into a dialogue or story and definite articles are used for nouns that already been introduced.
I read a book. The book is about a girl. The girl lives in a forest. The forest is known for untamed animals. The girl goes into the forest and encounters a raccoon. The raccoon is injured. The girl helps the raccoon, takes it home and they become friends.
In this example, the nouns book, girl, forest and raccoon take an indefinite article, but only when they are introduced for the first time. After they are introduced, we use the definite article in every instance.
There’s a visitor at our house. The visitor is a famous singer. The famous singer wants to help a poor family. The poor family he chooses to help is ours.
I ate an orange. The orange tastes very sweet.
Use the indefinite article a if the noun starts with consonant, and an if the noun starts with vowel.
2.When a Noun is Unique, Use a Definite Article.
Another important rule is when something is unique* or, in other words, there is only one of that object. In this case, the definite article is used.
A: Who is your father talking to?
B: He is talking to the president of the United States.
A: What book are you reading?
B: I’m reading the Principle of relativity.
This is especially true for objects that are well known by many or most people, but it is true even when the hearer may not know the object.
The uniqueness can also come by association:
A house was on fire. Fortunately, the family was able to escape safely.
Once we introduced the house, there could only be one family so the family was unique at that time of introduction and we use the driver (definite article) instead of a driver (indefinite article).
3. Use the Definite Article before a ” Superlative Expression.”
The prettiest girl.
My Science teacher is the most interesting person I’ve ever met.
4. When we are speaking of a noun in general we usually leave the article out and, if it is countable, use the plural form.
When we are talking about an actual object that really exist or existed, we use an article (definite or indefinite).
I drink a coffee today.
The coffee tastes really delicious.
In the above examples, we are speaking of real/actual/specific coffee.
However, we can also make general statements about coffee. And when we do speak of general of coffee, we leave the articles out:
I like coffee.
I drink coffee everyday.
Coffee can give me energy boost.
So, it is possible to either use an article or leave it out, but the meaning will be different in each case:
I like coffee.
(A general statement about coffee. We are talking about any coffee in this statement.)
I like the coffee.
(A statement about some specific coffee that the speaker has actually tasted.)