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"Una" is a form of "una", a noun which is often translated as "one". "Dos veces a la semana" is a phrase which is often translated as "twice a week". Learn more about the difference between "una" and "dos veces a la semana" below.
la una()
A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (time)
dos veces a la semana()
A phrase is a group of words commonly used together (e.g., once upon a time).