In English we sometimes use adjectives as nouns, e.g., "the rich and the poor," and German does the same. In both languages, essentially the adjective is standing for a missing, unspecified noun which this adjective would be modifying. For example: "the rich (people) and the poor (people)" or "I’ll take the blue (one) and the green (one)". But you’ll note two differences in how such words appear in German: they are capitalized like all other nouns, and they also carry the meaningful adjectival ending they would have as if they were modifying a noun. For example, in nominative case, referring to the plural idea of "rich (people)" and "poor (people)": "the rich" = die Reichen, "the poor" = die Armen. Such adjectival nouns are far more common in German than in English. Here are several more examples:
der Alte (nominative case, singular) “the old man” or some other masculine being, depending on context
die Alte (nominative or accusative case, singular) “the old woman” or some other female being, depending on context
einen Grünen (accusative case, singular) could refer to “a person associated with the Green party” or in a more general context, simply “a green one”
das Alte (nominative or accusative case, singular) “the old one” (given the neuter gender, it presumably refers to an object) or more abstractly, “that which is old”
das Gebaute (nominative or accusative case, singular) “that which was built” or "what was built" or perhaps "the buildings" – as always, adjust to the context
das Beste an der Sache (nominative or accusative case, singular) "the best part of the affair/story" or "what was best about this" (where Sache would be referring back to something previously explained)
As you can see, you must have mastered the meanings of the adjectival endings you learned in section 3 above, and you will need to pay attention to the form of any definite or indefinite articles which belong to this noun, in order to figure out this noun’s role and number within the sentence.
Also keep in mind that such nouns would be modified by adverbs rather than by other adjectives, in agreement with what you learned in the preceding section, and like the adjectival nouns you learned about in Unit 2. The third example below includes this situation.
Ich tue immer mein Bestes.
I always do my best.
Vermeers Gemälde zeigen oft das Schöne in dem Häuslichen.
Vermeer’s paintings often reveal the beautiful in the household sphere.
Ich suche immer stark Überraschendes als Geburtstagsgeschenk für meine Schwester.
I always look for something really surprising as a birthday present for my sister. [or:]
I always look for really surprising things as a birthday present for my sister.