¡La Esquina de la Gramática! Rule 26
“Sino” and “si no”. There is a difference between them!
The form “si no” (with a space in between) always introduces a conditional sentence. The word “si” is a conjunction (which introduces a conditional formula), and “no” is a negative adverb. Example:
Si no hubiera intervenido el juez…
The form “sino” (no space in between) is an adversative conjunction, which is mainly used to set against an affirmation to something previously denied:
No fue expulsado (what is denied), sino que dimitió (the affirmation).
This last form is also used to join two statements where the second one adds on something to the first one:
No solo se han visto ovnis, sino que además han aterrizado (Se han visto + han aterrizado).