Numbers 1-20 & Gender Agreement

Only three cardinals inflect for gender, and you already know one of them. 1 is ένας / μία / ένα - the indefinite article itself, with the masculine accusative έναν. 3 is τρεις for masculine and feminine, τρία for neuter. 4 is τέσσερις and τέσσερα the same way. Everything else - δύο, and πέντε through είκοσι - is indeclinable and never changes for anything at all. The teens compound as δέκα plus the neuter unit: δεκατρία, δεκατέσσερα, δεκαπέντε. And everyday Greek prefers the doublets εφτά, οχτώ, εννιά over the formal επτά / οκτώ / εννέα; both are correct, but the first set is what you will hear.