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"Cuarto" is an adjective which is often translated as "fourth", and "son las diez" is a phrase which is often translated as "it's ten o'clock". Learn more about the difference between "cuarto" and "son las diez" below.
cuarto, el cuarto(
kwahr
-
toh
)An adjective is a word that describes a noun (e.g., the big dog).
a. fourth 
Entra al edificio y toma el ascensor al cuarto piso.Enter the building and take the elevator to the fourth floor.
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
son las diez(  
sohn
lahs
dyehs
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
a. it's ten o'clock 
Son las diez, ¿qué haces todavía en la cama? - Hoy no tengo clase hasta las once.It's ten o'clock. What are you doing still in bed? - I don't have any class until eleven today.