Spanish language Q&A Forum

I am confused after getting an answer wrong in a test and then reviewing the subsequent lesson.   

In a recent test, this sentence was given to complete.   "No me gusto nada hacer ejercicio _____________."  My answer was "después de comer," when in fact the answer was "después de comiendo."   

I went back to review the lesson and the lesson states that after the preposition "de" comes the infinitive form of the verb to say the "ing" form of the verb.    Now I am confused as to why my answer of "después de comer" is not correct.   When do you use the infinitive?  and when do you use the present participle (i.e. ending "ando" or "iendo") after the preposition de?

Que Onda

This is one of the nuances of usted that I still haven't quite figured out. At my retail job, I often assist Spanish-speaking customers. However, I am not sure if these situations warrant using more formal language. In English, I address my customers politely with "Sir" or "Ma'am" but the language I use otherwise as I'm assisting them is more informal. I want to maintain the same tone of politeness yet casualness in Spanish as English but I don't know if it comes across as too formal. For context, I am in my early twenties and the customers I've spoken to are almost always older than me ranging from their thirties to more elderly people. Obviously, for my older customers, I would use usted but would it be necessary to use usted for people who are not that much older than me? I don't know if there is anybody here who can shed some light on this topic. In Spanish-speaking countries do retail employees typically address their customers with more formal, usted language?

Thank You

Nathan

Only the first word is being spoken in this example.

When you say that we use  "por culpa de" for something  with a negative result and use the example as given above " por culpa de mi novio soy la mujer más feliz del mundo........" Why do you say that this sentence has a negative result?