Help with Spanish Preterite and Imperfect Tense?

Dear I have studied Spanish for most of my life.Of course I know a lot of Spanish.To me it is one of my accomplishments in my old age to be able to speak,write and read Spanish.Understanding the spoken language is harder especially since I am going deaf.I regret to say that after so many years I know so little and have so much to learn.I get preterite and imperfect mixed up all the time.Your sentence; cuando estaba haciendo este pas� esta cosa. Maybe someone gives you a better sentence.