Only when the adjective is placed BEFORE A NOUN it get some endings. Otherwise (when it is a part of the predicate) it remains it its basic form: Die Frau ist schön. <-> die schöne Frau ist Model.
We are going to take a closer look to the case when the adjective stands before a noun and the logic behind the endings that it gets. Let’s make one thing clear: adjective builds one logical and grammatical unit with the word that stands before it and the noun that stands behind it. As we know, before the adjective can be placed either nothing (Null article) or the definite or indefinite article. Since those words vary in their “informative” value, the endings of the adjective will also defer. Let me explain what that means: if there is no word before the adjective, that means that the ending of the adjective will have to be very informative and provide the information about: the gender of the noun (that stands behind the adjective), the number of that noun (singular/plural) and the case (Nominative / Genitive / Dative / Akusative). In that case, the adjective gets the endings of the definite article. On the other hand, when definite article stands before the adjective, since it is very informative, the endings of the adjective do not have to be informative, and the adjective gets only –e or –en. Now when we have cleared that out, it will be much easier to memorize the numerous endings in the declination of Adjectives. My recommendation is: always learn the endings with the word before it, because the logic behind the whole story becomes clear that way.
Let me show you how the endings of the adjective look like in a “sandwich” (after the word that stands before it and the noun that stands behind it):
Other words that can appear instead of definite article: dieser, diese, dieses, diese; jeder, jede, jedes, alle; mancher, manche, manches, manche.
Other words that can appear instead of indefinite article: possessiv pronoun (mein, dein, sein, ihr, unser, euer, ihr).