¿ Por qué o para qué ?

Question

¿ Por qué o para qué ?

My grammar resources draw a distinction between:

¿ Por qué quieres llamar a Juan ?

Y

¿ Para qué quieres llamar a Juan ?

The English translation is the same for both, but which you should use depends on the answer given (Porque tengo que decirle algo / Para hablar con él).

How can I know which to use if I don't already know the answer ?

Answer 1

While they seem the same, they can have different nuances and the tone of voice is slightly different for each one.

¿Por qué? is the same as Why?

¿Para qué? is the same as What for?

¿Para qué? can also be used to indicate that the speaker thinks the subject asked about is silly, stupid, etc, ie: ¿Con tanto que pasa hoy en Irak, para qué quieres ir allá?

Answer 2

Es lo mismo, tu respondes a como te pregunten.

Answer 3

Maybe as follows

Why do you want to call Ed?

And

What do you want to call Ed for?

I am not a native speakers so wait for one to come along.