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"La cual" is a phrase which is often translated as "that", and "por la cual" is a phrase which is often translated as "why". Learn more about the difference between "la cual" and "por la cual" below.
la cual
A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
a. that  (things, animals)
Esta es la serie de TV sobre la cual te hablé. ¡Es buenísima!This is the TV show that I told you about. It's really good!
b. which  (things, animals)
Ordenamos la sopa de Spam, la cual se veía buena en la foto, pero resultó ser asquerosa.We ordered the Spam soup, which looked good in the picture, but it turned out to be disgusting.
por la cual(  
pohr
lah
kwahl
)A phrase is a group of words commonly used together (e.g., once upon a time).
a. by which 
El internet es la vía por la cual los miembros del equipo se comunican todos los días.The internet is the means by which the team members communicate with each other every day.
b. through which 
La aorta es la arteria por la cual pasa la sangre al resto de las arterias.The aorta is the artery through which the blood gets to the rest of the arteries.