Chloe, my eldest granddaughter, is set on improving her Spanish and thinks I can help her. She has a genetic knack for the language and yet la mano, el pie, el dedo, la muñeca, baffle her. And rightly so! I have decided not to mention genders, masculine, femenine or neuter. And certainly I will never tell her that Spanish has "masculino, femenino, neutro, común, epiceno y ambiguo."
I have simplified this business by pointing out that certain words use "el": el alma, el casco, el hampa, and others use "la": la alberca, la desgracia, la taxista. And I add that some other words use "lo": lo bueno, lo cortés, lo rico." And that is the way the cookie crumbles, I may add.
What we read is "el libro." What we buy with in England is "la libra."
I never explain rules or what-have you. Neither rhyme nor reason governs the idiom of a people, as Pearsall-Smith, the Quaker, put it. What I do with Chloe, at this stage, is correct her and correct her again, and again, and remind her to remember.
Knowing her, I know that soon she will be bilingual, like many of us in the family.