There is no formula. The thing to keep in mind is that "the" is more specific than "a.". For example, "the boy is taller than the girl." This requires a specific reference. Perhaps the boy and girl are standing near you. Or perhaps you are referring to the boy and girl you saw yesterday. "A" is much more general. For example, if we say "a boy is usually taller than a girl, the reference is very general. You are not looking at a boy or girl or even thinking of a specific boy or girl but are generalizing, based on your experience of seeing boys and girls in your village or city or in other parts of the country or the world
As a generalisation the above is roughly true, but there are clear rules. However, there are many of them and it can be, and is confusing for language learners!
Below are
some basic rules (which add some flesh to the generalisation above)
BASIC RULES:
1. A singular countable noun
must have an article (or other determiner) before it:
She gave me
a book. (I looked at the / that / this / his / John's computer)
I bought
the book which you recommended.
2. If you identify which specific thing you are talking about, then you must use the definite article
- this is true for singular, plural, countable, uncountable and abstract nouns:
She gave me a book.
The book she gave me is very interesting.
I like science books.
The science books in the university library are excellent.
I like rice.
The rice in that bag is dirty.
Love is a wonderful thing!
The love which they have for each other is very special!
3.
Generalisations: If you
don't identify specifically which thing you are talking and are only talking generally about a subject, then these are the rules...
for countable singular nouns you use the indefinite article,
a, an - I saw
an accident.
for countable plural nouns you don't use an article - I love books.
for uncountable or abstract nouns you don't use an article - I like sugar in my tea. I love life!
4. If you talk about something that is already known to the listener or reader, then we use
the definite article -
the (This is the same as rule 2. The object is already known to the speaker and listener.)
Put that chair in
the sitting room.
Don't look at
the sun!
Where did you put
the sugar?
Please open
the window. (How many windows are there in this situation?)
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We use the definite article with the idea of ‘possession' - with the preposition ‘of':
What is
the colour
of his car. T
he government
of Australia is in crisis.
The price
of that car is too high.
The length
of the room is 5 metres.