Sometimes adjectives become substantivized. In this case they have the functions of nouns in the sentence and are always preceded by the definite article. They can be partially substantivized (i.e. acquiring only some of the morphological characteristics of nouns) or fully substantivized (i.e. can be used with all articles).
Substantivized adjectives may have two meanings:
1) Substantivized adjectives may indicate a class of persons in a general sense (e.g. the poor = poor people, the dead = dead people, etc.)
Such adjectives are plural in meaning and take a plural verb. The old receive pensions.
If we wish to denote a single person, we must add a noun. The old man receives a pension.
If we wish to refer to a particular group of people (not the whole class), it is also necessary to add a noun. The young are usually intolerant. The young men are fishing.
Some adjectives denoting nationalities (e.g. English, French, Dutch) are used in the same way.
The English are great lovers of tea. There were a few English people among the tourists.
2) Substantivized adjectives may also indicate an abstract notion. Then they are singular in meaning and take a singular verb. The good in him overweighs the bad.