After an ein-word (ein, kein, or a possessive: mein, dein, sein, ihr, unser, euer, Ihr), the adjective takes the "mixed" ending. It's mostly the same as the weak pattern from der-words — but in three spots the ein-word itself doesn't show gender (ein, kein, mein, ... have no ending in masculine/neuter nominative and neuter accusative), so the adjective has to carry that information instead.

Quick test: this is the weak table from the definite-article lesson, except masculine nominative becomes -er and neuter nominative/accusative become -es — the endings der, das would have had if they were there. There's no plural ein, so kein/mein/etc. in the plural just borrow the weak -en pattern.