1. THE INDEFINITE ARTICLE
Use an before vowels: an (but not when ‘u’ or ‘e’ produces a ‘y’ sound: a useful tool, a European student, a university). When ‘h’ is silent, use an: an hour, an honest man, etc.
A or an is used:
with singular countable nouns mentioned for the first time:
A blue car came round the corner.
A strange man with a black beard walked through the door.
to express rates:
He drove at 50 kilometres an hour.
She earns $100,000 a year.
The indefinite article is not used with uncountable nouns or plural countable nouns:
More women go to university in Spain than man.
Knowledge makes people powerful.

2. THE DEFINITE ARTICLE The is used:
with things we have mentioned before or it’s clear who or what we are referring to from the context:
I’ve got a new teacher. The teacher is from California.
Could you go to the bank for me, please? (i.e. the bank we always use)
with things which are unique:
the internet, the moon, the sun
with adjectives to express groups:
In this country, the rich are growing richer and the poor are growing poorer.
with nationalities:
the French, the Spanish, the Italians
(Note: Nationality adjectives ending in –sh, -ch, -ese, and –ss have a singular form but are plural in meaning: the English drink a lot of tea, the Chinese are very hard-working. Other nationality adjectives have a plural form and a plural meaning: I think the Brazilians are going to win the World Cup again.)

3. with superlatives:
the best, the longest, etc.
with the first, the second, the third used as adjectives:
Manolo won the first prize and Igor won the second.
with names of countries which include these words: republic, kingdom, united, union, emirates:
The United States of America, The Czech Republic, The United Kingdom, etc.
with names of rivers, mountain ranges, seas and oceans:
the Nile, the Alps, the Mediterranean, the Pacific

4. ZERO ARTICLES (OMISSION OF ARTICLES)
Do not use articles:
with proper, abstract, and material nouns:
London, honesty, love, ice
when talking in general and in plural:
Teachers are not paid enough.
I can’t imagine offices without computers.
with the names of days, holidays, seasons, months
What do you do on Sundays?
The exam is in June.

5. with the names of games and sports: He plays basketball.
with the names of meals when talking in general:
Breakfast is the most important meal for me.
BUT if we think of a food or specific meal, ‘the’ is used: The breakfast is ready.
with many common expressions:
in/to bed, to church, at home, in/to hospital, in/to prison, at/to school, at/to university, at/to work
He’s in bed.
I’m at university.
What time do you go to work?
She’s been taken to hospital.