(The) is used before singular or plural nouns:
The man the men the books the book the day the days
It is used when there is only one:
The earth the sun the moon the stars the president
It is used before the noun which was mentioned before ( mentioned a second time):
I saw a man yesterday.The man was very tall .We didn't put the in the first sentence because it is the first time we mention it but we did in the second because it is the second time we mention it.
The is used before the names of seas ,rivers ,oceans, chain of mountains and a group of islands:
the Nile river the pacific ocean the red sea The Alps The canaries
It is also used before the names consisting of noun + of +noun:
EX:The Capital of France is Paris.
-There are other uses I haven't mentioned.
Notice:
The isn't used before names of people:
John Soha Sean Ronaldo Hazard
It isn't used before countries ,cities or villages:
England Liverpool Italy Rome
Notice: we use (the) with the countries which contain two or more names:
The USA The UK The czech Republic The UAE
-We also say The Sudan not Sudan
It isn't used before the names of meals:
EX:I had breakfast. Not the breakfast .
It isn't before the names of games:
EX:I play Tennis once a year.
The isn't used with some places when they are used for primary purpose:
EX:I go to hospital.(as a patient)
EX:I go to the hospital.(as a visitor)
EX:I go to church.(as a worshipper)
EX:I go to the church.(to see something)
EX:He went to prison.(as a prisoner)
EX:He went to the prison.(as a visitor)
EX:He went to school/university/college.(as a student)
EX:He went to the school/the university/the college.(as a visitor)