"Yesterday I was nervous, since I was going to meet the king."
Translation:I går var jag nervös eftersom jag skulle träffa kungen.
eftersom is a conjunction that does not require the subject-verb inversion.
Check here: http://forum.wordreference.com/showthread.php?t=1879658
I believe in both Swedish and English it’s optional, but you put it there to show that the first part is a main clause and the second part a subordinate clause. In languages like German and Faroese, it’s mandatory with the comma there, and used to be in Swedish as well some 50 years ago.
I går is an adverb, it describes when the verb took place. Think of it this way - if the sentence makes sense on its own, it is a main clause and the verb comes second: i går var jag nervös, (yesterday I was nervous) is perfectly valid on its own. När jag var nervös, (when I was nervous) doesn't make sense without an actual or implied context, so it's a subordinate clause and the verb comes after the subject.