When adjectives function as nouns denoting groups of people or things they are called substantivized adjectives. 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).
Note 1:When a substantivized adjective denotes a group of people (e.g. the rich, the wise, etc.), it is always in the plural. If we want to indicate a single person or a number of persons, we must add a noun.
The old man receives a pension.
The young manis fishing.
Note 2:Some adjectives denoting nationalities and ending in - (i)sh: British, English, Irish, Welsh; in -ch: Dutch, French and in -ese: Chinese, Japanese, Portuguese and the adjective Swiss are used with the definite article to form a substantivized adjective in the plural: the English, the Japanese. In other cases we should use the + the plural form: the Canadians, the Russians, the Americans.
The Use of Articles with Abstract Nouns | The Use of Articles with Material Nouns | The Use of Articles with Predicative Nouns and Nouns in Apposition | Articles with Names of Seasons and Parts of the Day | Articles with Names of Meals | Names of Persons | Geographical Names | Calendar Items | THE ADJECTIVE | Patterns of Comparison |