used before nouns when the object or person in question has been previously referred to or is obvious from the circumstances
Put the cat out
She ordered bread and cheese, but didn't eat the cheese
used to indicate that the noun that follows it refers to somebody or something unique or universally recognized
the pope
the south
the future
used to limit the application of a noun to that specified by what follows
the University of London
the man on my right
I didn't have the time to write
used before the name of a familiar feature of daily life
We talked on the telephone
I turned off the gas
The postman is late today
used before a noun denoting a period of time to indicate the present one or the one under consideration
book of the month
We left on the Monday
used before a singular noun to indicate generic use
a history of the novel
used before certain proper names
the Mayflower
the Rhine
used in the distinguishing titles given to some people, esp monarchs
George the Fourth
Ethelred the Unready
Brueghel the Elder
used before the names of parts of the body, clothing, etc instead of a possessive adjective
inflammation of the bladder
She took him by the sleeve
informal used before a noun denoting a member of one's family instead of a possessive adjective
The wife's been a bit poorly recently
used emphatically to designate one of a class as the best, most famous, etc
This is the life
You can't be the Elvis Presley!
used in prepositional phrases to indicate that the following noun serves as a basis for computation
sold by the kilo
Eggs are sold by the dozen
used to transform an adjective or participle into a noun
I can't stand the waiting
the British