Sí와 Si의 차이

Spanish "sí" with an accent

1. An adverb that expresses affirmation. This usage is usually translated as yes.

- ¿Quieres una cerveza? - Sí, gracias.- Would you like a beer? - Yes, thank you.

Spanish "si" without an accent

1. Si as a conjunction that introduces conditional sentences. This usage is usually translated as if.

Si quieres, nos podemos tomar unas cervezas.If you want, we can have some beers.

2. Si as a conjunction in indirect questions. This usage is usually translated as whether. 

No sabe si prefiere español o francés.She doesn't know whether she prefers Spanish or French.

Pregúntale a Víctor si va a venir al cine o no.Ask Víctor whether he is coming to the cinema or not.

cf. Bear in mind that, although these are the most common translations for Sí and Si, they could also refer to:

• "yes" as a noun meaning "approval", e.g "Ella le dio el sí en el altar" (She gave him the yes/her approval/her assent at the altar.)
• the musical note B, the  seventh in the scale, i.e "Do, re, mi, fa, sol, la, si, do"
• the emphatic reflexive pronoun "sí", e.g. "María se critica mucho a sí misma" (María criticizes hersef a lot.)