In the lyrics of the song "Un fachiro al cinema" by Paolo Conte, we see "passasse" (congiuntivo imperfetto) occur as the main verb of the first sentence:
Passasse il sole lontanamente da qui
Sopra le nebbie dell'arte! Io sì
Come un fachiro mi stiro e rigiro
E mi storco e contorco
Ti guardo e non guardo più…
Is my guess correct that an unrealised wish is implied -- that is, "(I wish that) the sun were passing far away from here"?
Here the imperfect subjunctive is used to express a strong desire or a desire that cannot possibly be fulfilled. You can translate the sentence as "if only the sun could pass..." / "I wish the sun were passing...".
Imperfect subjunctive is also used to express a possibility that is unlikely to happen (but not necessarily impossible), impossible to happen or is not happening now (but could happen in the future). In this case, often the "if" ("se") is implied:
"(Se) Studiassi con più impegno, i tuoi voti sarebbero migliori" = "(If) You studied harder, your grades would be better".
"(Se) Fossi in te, non lo farei" = "(If) I were you, I would not do it"
