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QUICK ANSWER
"Entre sí" is a form of "entre sí", a phrase which is often translated as "among themselves". "Ellos" is a form of "ellos", a pronoun which is often translated as "they". Learn more about the difference between "entre sí" and "ellos" below.
entre sí(
ehn
-
treh
see
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
a. among themselves
Los delfines se comunican entre sí mediante sonidos.Dolphins communicate among themselves through sounds.
b. to each other
Clara y Jorge llevan un rato cuchicheando entre sí. Creo que traman algo.Clara and Jorge have been whispering to each other for a while now. I think they're up to something.
c. each other
Los cuatro amigos siempre se han ayudado mucho entre sí.The four friends have always helped each other a lot.
d. no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Las dos autopistas están conectadas entre sí por una carretera.The two freeways are connected by a road.
Los dos cuartos están comunicados entre sí por una puerta.The two rooms are connected through a door.
ellos(
eh
-
yohs
)A pronoun is a word that stands in for a noun (e.g., she).
b. them (after prepositions)
Si confías en ellos, dales las llaves.If you trust them, give them the keys.
c. them (in comparisons)
Mi hermano y yo somos tan altos como ellos.My brother and I are as tall as them.
