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"Eso" is a form of "eso", a pronoun which is often translated as "that". "Si" is a form of "si", a conjunction which is often translated as "if". Learn more about the difference between "eso" and "si" below.
eso()
A pronoun is a word that stands in for a noun (e.g., she).
si()
A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
