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"Cuarto" is an adjective which is often translated as "fourth", and "a las ocho" is a phrase which is often translated as "at eight". Learn more about the difference between "cuarto" and "a las ocho" 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).
a las ocho(  
ah
lahs
oh
-
choh
)A phrase is a group of words commonly used together (e.g., once upon a time).
b. at eight o'clock 
El vuelo a Buenos Aires sale a las ocho de la mañana.The flight to Buenos Aires departs at eight o'clock in the morning.