Hi everyone!
A few days ago @Sacharbonne asked me in a live lesson when to use por and para, at that time I had many doubts about the specific rules of use of these prepositions, so, I decided to write a new post explaining this for @Sacharbonne and all the other chatterbug students who may need it 
Corrections, more examples, more uses for these prepositions and doubts are very welcome!
Por is used to:
- Express movement:
Paseamos por la ciudad (We walked around the city)
- The cause or reason for an action:
Tuve un accidente por falta de atención (I had an accident due to lack of attention)
- Means of travel and communication. (It can refer to the way of travel, such as train, car, etc. or communication (phone, message, email, etc.):
Hablé con él por teléfono ( I spoke to him on the phone)
Viajé por Italia. (I traveled through Italy)
- To indicate the time or duration when something ocurrs:
Me gusta hacer deporte por la mañana. (I like to do sports in the morning)
- To talk about exchanges:
Gracias por el regalo (Thanks for the gift)
Te cambio mi comida por tu bebida. (I exchange my food for your drink)
Para is used to:
- Indicate destination:
Voy para Madrid mañana. (I am going to Madrid mañana)
- To indicate the recipient of something:
Este reloj es para mi hermana. (This watch is for my sister)
Esta comida es para gatos. (This food is for cats)
- To talk about deadlines or a specific time:
Tengo que terminar este post para las 8pm. (I have to finish this post by 8pm)
Necesito una cita médica para el lunes. (I need a doctor´s appointment for Monday)
- It´s also used for goals or purposes:
Como fruta y verdura para estar saludable. (I eat fruits and veggies to stay healthy)
Hago ejercicio para adelgazar. (I exercise to lose weight)