How do you know which preposition to use with the infinitive? It seems sometimes they use "para" sometimes the use "que" and sometimes "a"?
"para" is often used to mean "to" in the sense of "in-order-to". "que" is often used in set expressions pertaining to having to do something, as in tener que, haber que. There are several categories of verbs that require "a" when you use an infinitive, which you just have to memorize. Among them are verbs of learning/teaching (e.g ensenar, aprender), verbs of motion (e.g acercarse), verbs of becoming/beggining (e.g. ponerse, empezar). Full list here: http://www.elearnspanishlanguage.com/grammar/verb/verbswithprep-a.html