Adjectives: agreement and position τὰ ἐπίθετα

An adjective describes a noun, and it agrees with it: it takes the same gender, number and case. So an adjective has a full set of forms for each gender.

Good news: you know these endings already. The commonest adjectives use the second declension for the masculine and neuter (like λόγος and δῶρον) and the first declension for the feminine (like ψυχή). Dictionaries list all three: ἀγαθός, ἀγαθή, ἀγαθόν.

Position. With the article, where the adjective stands matters. Inside the article's phrase (between the article and the noun, or after the noun with the article repeated) it simply describes: this is the attributive position, "the good man". Outside it, the adjective makes a statement: this is the predicate position, "the man is good". Greek can leave out ἐστί "is" in a statement like this.