QUICK ANSWER
"Si" is a form of "si", a conjunction which is often translated as "if". "Yo sí" is a phrase which is often translated as "I do". Learn more about the difference between "yo sí" and "si" below.
yo sí(
yoh
see
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
b. I did
No vi la película. - Yo sí que vi la película y me pareció fantástica.I didn't see the film. - I did see the film and I thought it was fantastic.
e. I could
¿Podríais venir mañana a cenar? - Supongo que yo sí, pero no sé si mi esposo podrá.Could you come tomorrow for dinner? - I guess I could, but I don't know about my husband.
f. I am
¿No estás viendo la serie de terror sobre zombis? Pues yo sí.Aren't you watching the horror series about zombies? Well I am.
g. I was
Mi hermano Luis no estuvo en esta escuela de pequeño, pero yo sí.My brother Luis wasn't in this school as a kid, but I was.
si(
see
)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).
