vs
QUICK ANSWER
"Si" is a form of "si", a conjunction which is often translated as "if". "Eso" is a form of "eso", a pronoun which is often translated as "that". Learn more about the difference between "si" and "eso" below.
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).
eso()
A pronoun is a word that stands in for a noun (e.g., she).