Por and Para: a closer look

Por and Para came from the same Latin word ("pro" originally, but soon split into "por" and "por a"  which eventually became "por" and "para".   You can often sustitute "hacia" for "para"
Por is used:
  • to indicate a reason or cause: Lo hizo por bueno (He did it for [the common] good). Esto es por ti (this is because of/due to you [your fault].)
    • Con verbos de movimiento: Voy por agua (I went for [to get] water)
  • to indicate direction or path
    •  con verbos de movimiento: Voy por Toledo (I went through or in the direction of Toledo); Voy por el norte (I went north).
  • to identify an agent or person: Lo hizo por ti (He did it because of you) ; Fue construído por Mauricio Izquierdo (It was built by Mauricio Izquierdo). 
  • to indicate a period of time: Será por agosto (it will be in August, during August)
  • substitution: Pago por algo (I pay for something); Doy el libro por el otro (I give this book for the other [book].)
Para is used:
  • to indicate a destination or end: Voy para agosto (I'm going by August) (deadline).  Este regalo es para ti (this gift is for you).
    • Con verbos de movimiento: Voy para Toledo (I'm going to Toledo.) 
  • to indicate perspective: Para un joven, sabe mucho de lo político (for such a young guy, he knows a lot about politics).
  • to indicate a purpose: Voy para aprender más (I'm going in order to learn more).
¿Por qué lo hiciste?              Why did you do it? Motive, Cause
¿Para qué lo hiciste?            What was your intension? (to what end?) purpose, goal

Regreso por agosto.             I'll return in August.
Regreso para agosto.           I'll return by August.

Esto es por ti.                     This is because of you (your fault).
Esto es para ti.                   This is for you.

Visito por la Navidad.          I'll visit at Christmas.
Visito para la Navidad.        I'll visit for Christmas.