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QUICK ANSWER
"Cuál es" is a form of "cuál es", a phrase which is often translated as "which is". "No sé qué" is a phrase which is often translated as "I don't know what". Learn more about the difference between "no sé qué" and "cuál es" below.
no sé qué, el no sé qué(
noh
seh
keh
)A phrase is a group of words commonly used together (e.g., once upon a time).
a. I don't know what
No sé qué hacer para aprobar el examen de inglés. ¡No entiendo nada!I don't know what to do to pass the English test. I don't understand a thing!
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
a. a certain something
Le falta un no sé qué a la pintura. Algo que la haga memorable.A certain something's missing from the painting. Something that makes it memorable.
b. no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Tatiana tiene un no sé qué que la hace una mujer muy interesante.Tatiana has a je ne sais quoi that makes her a very interesting woman.
John tiene un no sé qué que hace a mi corazón latir más rápido.There's something about John that makes my heart beat faster.
cuál es(
kwahl
ehs
)A phrase is a group of words commonly used together (e.g., once upon a time).
a. which is
Laura quiere el vestido color durazno, no el salmón, pero no sé cuál es cuál. Los dos se ven iguales.Laura wants the peach-colored dress, not the salmon, but I don't know which is which. They both look the same.
b. which one is
¿Cuál es tu hermano? - El que tiene el suéter rojo.Which one is your brother? - The guy in the red sweater.
c. which ... is
Entre la torre Eiffel y la torre inclinada de Pisa, ¿cuál es más alta?Between the Eiffel Tower and the Leaning Tower of Pisa, which building is taller?
a. what ... is
Honestamente, no entiendo cuál es el problema.Honestly, I don't understand what the problem is.
