Definiteness: when the is needed

The does not mean "important" or "specific in my mind"; it signals that the listener can work out which one you mean. There are four common reasons why a noun is identifiable.

• mentioned before → a plan → the plan
• only one here → the chairman, the sun
• defined after it → the cost of the project
• part of a scene → a wedding → the bride

• We were offered a contract in May, but the contract was withdrawn in June.
• The chairman opened the meeting at nine.
• I bought a second-hand book and found that the title page was missing.

Generic reference: three ways to name a class

A generic statement is about all members of a class, not about particular ones. The three forms are not interchangeable: they differ in formality and in what kind of statement they allow.

• Antibiotics are useless against viruses.
• The printing press transformed European society.
• A contract is a legally binding agreement.