Does anyone know how you differ from: yes (sí) from: if (si) when i'm a talking exam?

One has the accent - yes. Without the accent, it means, "if".

Explanation:

You can generally understand this through contextual clues. If there is a question being asked, typically the yes is followed by a comma.

For example:

Tienes amigos? - Sí, tengo muchos. (Yes, I have many (a lot)).

*If you have a reason or another situation being posed after it, then it is likely the if meaning. Typically, there would not be a comma after it, rather a clause of some sort.

Example:  Tienes amigos? -- Si me preguntas porque crees que no soy popular, entonces, no te voy a contestar.  (if you are asking because you think I'm not popular, then I am not going to answer you).

Or perhaps a conditional phrase if you have learned it...

Si tuviera más oportunidades para viajar (If I'd had more opportunities to travel...)

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