Ich spreche mit dem netten Mann.
English: I speak with the nice man.
Adjective declension in the dative case means changing the ending of an adjective when it describes a noun that is in the dative case in German. The ending depends on the gender, number, and article before the noun.
Use adjective declension in the dative case when you describe something (with an adjective) that is the indirect object in the sentence, usually after prepositions like 'mit', 'bei', 'nach', 'zu', 'aus', 'von', 'seit'.
Ich spreche mit dem netten Mann.
English: I speak with the nice man.
Sie geht mit der freundlichen Frau.
English: She goes with the friendly woman.
Wir spielen mit dem kleinen Kind.
English: We play with the small child.
Ich fahre mit den alten Freunden.
English: I travel with the old friends.