1. We have mentioned it before.
- He has a mother. The mother is sick.
2. We say which one we mean.
- The man with the yellow hat is George's friend.
3. It is clear from the situation which one we mean.
- Everybody went to the picnic.
- He spoke to the boss about getting a raise. (You know which one - my boss.)
- Please turn off the light when you're done. (You know that the light in the room is the one I'm talking about.)
B. There is only one around.
- The Black Death devastated Europe in the 14th century.
- People say that the moon is made of green cheese.
C. It concerns the physical environment.
1. The city, the country, the weather, the sea, the future, etc.
- Listen to the falling rain.
- Do you want to go camping in the mountains on your vacation, or would you rather rent a cottage by the seashore?
- Some people would rather live in the country instead of the city.
2. Used with superlatives—usually only one best, biggest, etc., so we know which one.
- That's the biggest park in our town.
3. Also used with first, next, last, same, only
- It was a coincidence that we got the same score on our tests.
- That's the last straw - I will never shop there again.
4. “the” = “the well-known”
- The President will give his State of the Union address tonight.
- Did you ever see Yo-Yo Ma, the cellist?
When “the” is NOT used:
A. Possessives and demonstratives.
- My father is an engineer. (Not The my father is an engineer.)
- Give me that book when you're done. (Not Give me that the book when you're done.)
B. Proper names.
- They went to Canada on their vacation. (Not They went to the Canada on their vacation.)
C. Things in general.
- Cats are popular pets. (Not The cats are popular pets.)