Let us today see a few more places where ‘the’ is used:

‘The’ is used before:
* superlatives ( third degree of adjectives )
the best day, the happiest moment, the most amazing experience

* numbers used as adjectives
the first row, the last week, the twelfth man

* ‘only’ used as an adjective
the only child, the only way

* singular noun that represents a class of animals or things or persons
The whale is on the verge of getting extinct.
The microwave has made work easier for women.
The first class traveller demands more comfort as he pays more.
( remember the verb following the noun is singular because the noun here is singular even though it represents a class or group )

* an adjective used to represent a class of persons
The old are quite weak and vulnerable.
The young are full of life.