Si soy (fuí) malo! Well, I am (was) wicked!
¡Si será (sería) tan necia! Can (could) she be so foolish!
¿Si vendría? I wonder will he come!
¡Si se lo había dicho yo mil veces! But I had told him so many times!
Other idiomatic uses of this si will be learnt by practice (all more or less pleonastic as in the above examples).
We have said that "but" is translated by sino after a negative unless a finite verb follows. Therefore, "Not to buy but to sell" is translated "No comprar sino vender."
After a negative a finite verb may be preceded by sino que instead of pero in cases like the following examples—
No compró sino que vendió: He did not buy but (on the contrary) he sold.
No sólo que es barato sino que es de muy buena calidad:
Not only is it cheap, but it is (also) of a very good quality.
Ni … ni—"neither … nor" (same as all negative words) when following a verb requires No to precede the verb, as—
No acepta ni esto ni aquello: He accepts neither this nor that.
But—Ni esto ni aquello quiere aceptar.
Pues—"seeing that" or "since" is used often for "then," "but," "well" (used as an interjection).
Pues que lo haga: Let him do it then.