Use the when both speaker and listener know which thing you mean (context, unique, obvious).
Use the + noun when it is definite (identifiable).
Do not use the for general plural/uncountable nouns (Cats are friendly; Water is essential).
Can the listener identify which one? If yes, use the.
💡 Definite does not always mean 'mentioned before'. Context can make it definite (Close the window — the window in this room).
✓Can you close the window?
✗Can you close a window?
In the room, it is clear which window is meant.
✓I left my bag in the car.
✗I left my bag in a car.
Both people know which car (the one you used).
✓The sun is bright today.
There is only one sun (unique reference).