In a family, there is one husband and one wife. We therefore know which husband and which wife is meant. If you used 'a husband' and 'a wife' it means that the family contains more than one of these and you are referring to one out of many.

That is a very interesting question, and the other respondents have given great answers. However....I would likely use phrases that are a bit less stereotypical in nature. It is commendable that you are studying English, and I think you have a good command of the language.

My name is Marisa, if you want, we can change Spanish classes for English classes. I have the afternoons available. Me llamo Marisa si quieres cambiamos clases de español por clases de ingles. tengo disponibles las tardes