Explain the usage of the definite article 'the'.

The definite article 'the' is used to refer to a specific noun or noun phrase that is already known or has been previously mentioned. It is used when the speaker and the listener both know which particular thing or things are being referred to.

Here are some specific instances where 'the' is used:

1. When referring to a specific noun that is unique or one-of-a-kind:
- "The sun is shining brightly today."
- "I saw the Eiffel Tower during my trip to Paris."

2. When referring to a noun that has been previously mentioned:
- "I bought a new car. The car is red."
- "There is a dog in the park. The dog is playing with a ball."

3. When referring to a noun that is known to both the speaker and the listener:
- "Let's go to the cinema. The movie starts at 7 pm."
- "I need to buy some milk. Can you pass me the carton?"

4. When referring to a noun that is unique within a specific context:
- "The president of the United States is visiting our city."
- "The book you recommended was really interesting."

5. When referring to a noun that is a specific group or category:
- "The elderly need special care and attention."
- "I love listening to the music of the 80s."

It is important to note that 'the' is not used with non-specific or general nouns. In such cases, indefinite articles like 'a' or 'an' are used. The usage of 'the' can vary depending on the context and the speaker's intention to specify a particular noun.