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"Sobre" is a form of "sobre", a preposition which is often translated as "on". "Por" is a preposition which is often translated as "by". Learn more about the difference between "sobre" and "por" below.
sobre(
soh
-
breh
)A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
5. (cover)
a. envelope 
Abrió rápidamente el sobre y sacó la carta de su interior.He tore open the envelope and pulled out the letter inside.
6. (colloquial) (bed) (Argentina) (Spain)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Regionalism used in Argentina
Regionalism used in Spain
a. sack  (colloquial)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
En cuanto llegué, me metí en el sobre.As soon as I arrived, I hit the sack.
a. clutch bag 
Necesito un sobre que combine con mis zapatos plateados.I need a clutch bag to match my silver shoes.
b. clutch 
Mariela suele llevar un sobre cuando asiste a una boda.Mariela usually carries a clutch when she attends a wedding.
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.