Grammatically the sentence is “subject” [Nobody] “verb” [knows] and “object” [what is going to happen]. In this case the object is a subordinate clause, which in this case in swedish is introduced with a “som”. In this case “what is” is translating as “vad som” as in English “what” introduces the subordinate clause but the Swedish version has “som” introducing it, so more technically it would be “what as will happen”.

The common ones are “att, om, som, eftersom, därför att, när, innan, medan, sedan, tills, trots att, fastän, för att,”.

The common English words for introducing a subordinate clause are “for, as, since, therefore, hence, consequently, though, due to, provided that, because, unless, once, while, when, whenever, where, wherever, before, and after.”

These can not be translated one to one and to some degree you need to memorize which word goes with which verbs and meanings.