One small word, one big difference in meaning
Functions and examples
1. We use a / an when we are talking about something for the first time.
He is wearing a suit and a tie.
They have a cat.
2. We use the when we are talking about a specific person or thing, or if there is only one, or if it is clear which one we are talking about.
She took a glass of water and started to drink. (one of many glasses of water)
She took the glass of water nearest to her. (a specific glass)
She took the glass of water and started to drink. (there is only one glass of water)
Can you turn on the TV? (it's clear we are talking about the TV in this room)
I have to see the doctor tomorrow. (it's clear I am talking about my usual doctor)
I need to see a doctor. (not any specific doctor)
How many people have walked on the moon? (there is only one moon)
The best film I've ever seen is Top Gun. (there can only be one best film)
3. We use no article when we are talking about people or things in general.
Carrots are good for you. (carrots in general)
The carrots growing in my garden are almost ready to eat. (specific carrots)
English people drink a lot of tea. (English people in general)
The English people in this hotel are very nice. (specific English people)
4. We use a / an to say what kind of person or thing someone or something is (often with an adjective, or to say it belongs to a particular group.
You have a nice house.
That's a very expensive car!
A cat is an animal a bus is a vehicle.
5. With school, university, prison, hospital, church, bed, work and home we use the when we are talking about a particular one, and no article when we are talking about the idea of school, university...
The church on our street was built in the 17th Century. (a particular church)
I go to church every Sunday (the idea of church, not a particular building)
My friend works at the prison outside the city centre.
My husband is in prison.
The work isn't finished yet.
I was feeling sick yesterday so I didn't go to work.
The new bed is bigger than the old one.
I put my children to bed a 8pm.
6. We use the with singular countable nouns to talk about a type of thing.
The computer was invented in the 20th Century. (not a particular computer)
The computer in my office is broken. (a particular one)
The crocodile is very dangerous. (this type of animal)
The crocodile in the river is very big. (a particular one)
She plays the piano. (not a particular piano but this type of instrument)
The piano she is playing is very old. (a particular one)
7. We use the + adjective to talk about a group of people (including nationalities).
I always give money to the homeless.
The unemployed receive money from the government.
The French produce good wine.
8. We use a / an to talk about someone's job.
My wife is an optician.
She works as a mechanic.
9. We use no article with continents, countries, regions, cities, streets, mountains, lakes and parks.
Asia, Italy, California, Bristol, Main St., Mount Everest, Lake Superior, Central Park
We use the if the country contains Kingdom, Republic, State, Union
10. We use the with plural names of people and places.
The Smiths, The Netherlands, The United States, The Bahamas, The Alps (also The Caribbean)
11. We use the with oceans, seas, rivers and canals.
The Atlantic Ocean, The Red Sea, The Nile, The Panama Canal
12. We use the with north, south, east and west to talk about the location of a place within another place, but no article to compare the location of two places.
Greece is in the south of Europe.
Spain is south of France.
We use no article with northern, southern, eastern and western:
Greece is in southern Europe.
13. We normally use the with buildings, except if the first word is the name of a place.
The White House, The Sheraton Hotel.
Oxford University, Buckingham Palace.