Spanish phrase
No es por ~ sino por ~.
Meaning
It is not because of ___ but because of ___.
When to use it
Denies a presumed cause and gives the real cause. Used to correct assumptions in explanations with a direct, matter-of-fact tone.
Example dialogue
¿Llegaste tarde por el tráfico?
Did you arrive late because of traffic?
No es por el tráfico, sino por un accidente en la carretera.
It is not because of the traffic but because of an accident on the road.