Adjectives are divided into three classes according to the presence/absence of an article. But I don't understand why a certain adjective does not simply take the ending of the noun it refers to or that of the article. Why a separate declension for adjectives, which may be misleading (for ex. der liebe Vater and not der lieber Vater, which to me is more spontaneous) is necessary?
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This table shows an overview. You see if an article is used, it's not necessary to decline it again, the article does that already. But that's the only logical reason, it's historically developed.