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"La" is a definite article which is often translated as "the", and "por" is a preposition which is often translated as "by". Learn more about the difference between "la" and "por" below.
la(
lah
)A definite article is a word that identifies a noun as specific or known to the speaker (e.g., the girl).
a. the 
La pelota está en el tejado.The ball is on the roof.
La leche está en el refrigerador.The milk is in the refrigerator.
A pronoun is a word that stands in for a noun (e.g., she).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
por(
pohr
)A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
5. (by way of)
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
¿Has ido a por tu hijo a la escuela?Have you picked up your son from school?
Habrá que ir a por leche cuando lleguemos.We'll need to go out for milk when we get there.
8. (for each)
10. (in place of)
a. for 
Yo terminaré el trabajo por ti cuando estés de vacaciones.I'll finish the job for you while you're on vacation.
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Está por verse.That remains to be seen.
El asunto está por resolverse.The issue is yet to be resolved.