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"Las sillas" is a form of "silla", a noun which is often translated as "chair". "Detrás de" is a preposition which is often translated as "behind". Learn more about the difference between "las sillas" and "detrás de" below.
la silla(
see
-
yah
)
A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
feminine noun
a. chair
Me compré una silla muy cómoda para el escritorio.I bought a very comfortable chair for my desk.
b. seat
¿Esta silla está ocupada?Is this seat taken?
a. saddle
Su silla está hecha de cuero.Her saddle is made of leather.
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detrás de(
deh
-
trahs
 
deh
)
A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
preposition
a. behind
Los motivos detrás de sus acciones son claros.The motives behind her actions are clear.
b. after
¿Quién es Celia? - Es la muchacha detrás de Felipe en la fila.Who is Celia? - She's the girl after Felipe in line.
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