"My mother is a secretary."
Translation:Η μητέρα μου είναι γραμματέας.
Female nouns don't end just in -η or -α. It's just the most common of female endings. Female nouns ending in -ας (like γραμματέας), or -ος (like έξοδος, είσοδος) are way fewer and less common than the ones ending in -η or -α.
And yes. Female nouns ending in -ας do form the accusative in -α. ^.^
Γραμματέας has the same nominative and accusative forms for both masculine and feminine, but you have to use the correct article forms for male or female. Ένας is the masculine form of the indefinite article so it's like saying that your mother is a male secretary. You have to use the feminine form μια (or you can omit the indefinite article altogether).