¿Por qué?
Duration of time (por)
By a certain time (para)
By means of (por)
Per (por)
Thank you for always uses por.
An exchange uses por
Going to a destination uses para
Movement through or around uses por
Comparisons use para
To pick something up uses por
To work for someone. My boss (para)
To work for someone, in their place (por)
because of (por)
in order to, purpose (para)
intended for (para)