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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()
A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (furniture)
detrás de()
A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
