''Si'' is translated into various forms in clauses:
Si / So : I am so desperate = Je suis si desespere (')
Si / If : If it rains... = S'il (Si il) pleut
Si / As : It is not as easy... = Ce n'est pas si facile
(comparison)
Si / Whereas
Si / No matter how
Et si / What if/ how about
Si? / Did i hear you? : If I'm fine? = Si je vais bien?
Si / Yes : (a response to a negative question)
Aren't you ready yet? = Si Si (yes yes) I am