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QUICK ANSWER
"How nice" is a phrase which is often translated as "qué agradable", and "that" is a conjunction which is often translated as "que". Learn more about the difference between "how nice" and "that" below.
how nice()
A phrase is a group of words commonly used together (e.g., once upon a time).
a. qué agradable
Did you get the chance to see your mother? How nice.¿Tuviste la oportunidad de ver a tu madre? Qué agradable.
a. lo agradable
Remember how nice it is to make dinner at home?¿Recuerdas lo agradable que es preparar la cena en casa?
a. lo amable
Now that Pilar's started a charity, everyone can see how nice she is.Ahora que Pilar fundó una caridad, todo el mundo puede ver lo amable que es.
that()
A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
A pronoun is a word that stands in for a noun (e.g., she).
An adjective is a word that describes a noun (e.g., the big dog).