When to use se in a sentence in Spanish? I understand pronouns and reflexive verbs to some degree but sometimes I see se in a sentence and I do not understand the rules. For example, " Pero se puede aprender" Why cant it be written Tu puedes aprender or Usted puede aprender? Why is se necessary?
This is a little too broad of a question, but in your example it's because you could generalize it to anybody and it isn't specific to who you're talking to. Think of it as "one can learn" instead of "you personally/specifically can learn".
Depends that you can use those phrases. For example you can say "Tu puedes aprender cualquier idioma, aunque requiera de esfuerzo" o "Aprender un idioma requiere de esfuerzo, pero se puede"
To the best of my knowledge it's an impersonal way of saying "you/one." For example, if you were on holidays in a Spanish speaking country and you wanted to ask somebody "where can you/one change money?"
You could say: ¿Dónde se puede cambiar dinero?
The word "usted" is used in formal situtations: when you are talking to boss, parents or someone with a higher rang than yours, however, the word "tú" (personal pronoun) is used when you are talking with a person with the same level, for example cousin, sibligns, friends or if that person authorizes you.