The acts as a definite article, specifying a particular noun or something specific. Here are situations for using the before nouns.
- When the thing is already mentioned, or we know which thing we are talking about
 
Example
- A man and a woman sat in front of me. The man was British, but I think the woman wasn’t.
 - ‘Where are the kids?’ ‘They’re in the garden’.
 
- (=Both speakers know which kids and which garden).
 
- When the thing is made specific by some details we give to it
 
Example
- I sat on the chair in the corner.
 - I saw the man who tried to rob me.
 
- When there is only one of it: the earth, the moon, the planet, the internet, the universe, the sky, the city, etc.
 
Example
- I need to talk to the manager.
 - The moon looks beautiful today.
 
- When we talk about places where we commonly go: the park, the cinema, the doctor, etc.
 
Example
- I’m going to the bank.
 - I found Peter at the station.
 
- Superlatives
 
Example
- This is the best restaurant in town.
 
- Name of oceans, seas, rivers, continents, and canals
 
Example
- I’d like to cross the Atlantic on a cruise.
 
- Countries that are plural or that include words like State, Republic, or Kingdom
 
Example
- the US (the United States), the UK (the United Kingdom), the Philippines, the Netherlands, the Czech Republic.
 
References