Only when the adjective is placed BEFORE A NOUN (so to say, on its left side) it get some endings. Otherwise (when it is a part of the predicate i.e. when it is placed on the right side of the noun) it remains in its basic form:
die schöne Frau ist Model. <-> Die Frau ist schön/.
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.
Before the adjective can be placed either nothing (Null article) or the definite or indefinite article:
Ø / definite article / indefinite article + adjective + noun
Since articles vary in their “informative” value, the endings of the adjective will also differ. 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, 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 very 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 take into consideration the endings of the article when you learn the adjective endings, because the logic behind the whole story becomes much clearer that way.
Let me show you how the endings of the adjective look like in a “sandwich” i.e after the word that stands before it (and the noun that stands behind it), where yellow are the endings of the article, blue are endings of the adjective and green are endings of the noun:
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: kein, keine, kein and possessiv pronoun (mein, dein, sein, ihr, unser, euer, ihr).