Por / Para - Tricky cases

If you haven't read the theory on por and para, I would start there.
You can also watch the video tutorial on por and para here.

After that, it's good to see examples where the sentence is very similar, maybe identical, and depending on if you use por or para.

With certain "things"

Example:
He comprado una planta preciosa para la ventana - I have bought a gorgeous plant for the window.

He visto una planta preciosa por la ventana - I have seen a gorgeous plant through the window.

In these cases para, end goal (to put on that place) and with por through, it means physical path.

Example 2:
Necesito una funda para mi móvil - I need a case for my phone. Prefiero hablar por móvil - I prefer talking on the phone.

Again, para here is a purpuse or end goal (to use on the phone), and por is through, in this case "via phone".

With "places"

These cases are very similar, because por means through also in these examples, but more of a physical path.
Para here means the same as a, to introduce a destination.

Examples:
El tren va por Segovia - The train goes through Segovia. El tren va para Segovia - The train goes to Segovia. We could have used el tren va a Segovia.

Nunca paso por la calle Gran Vía - I never pass by Gran Via street. Voy para la calle Gran Vía - I go to Gran Via street.

With "people"

This is probably the most confusing one because the translation can be identical:

Example:
Voy a intetnar terminar un proyecto para mi jefe - I'm going to try to finish a proyect for my boss.

Voy a intentar terminar un proyecto por mi jefe - I'm going to try to finish a proyect for my boss.

In this case, para mi jefe means the proyect is for him, to give to him (using para for people receiving something). Por mi jefe, on the other hand, means because of him (using por as a reason, like doing a favour for someone).

These two can even happen in the same sentence:

Example:
Abro una cuenta para mi abuela / por mi abuela - I open an account for my grandma.

You can open an account that your grandma receives (para) but also you're doing something for someone, because of your grandma (por), even on behalf of her (por as well).

This topic takes some time and practice, but don't give up!
Start with the basic theory, the video and then this, don't leave too many days between sets, and then find examples in any text you read.

¡Hasta pronto! :)