Translation:If you don't have anything to do, please don't do it here.
Vennligst translates to English 'kindly', and is used in almost exactly the same way. You may see it occasionally on informal signs in both languages, but it's not commonly used in speech.
When it is used in speech in English, it often suggests some irritation, or even sarcasm, on the part of the speaker - e.g., 'Kindly remove your feet from my desk (before I remove them for you)' - so it should be used with extreme care. I'd be curious to know if vennligst carries the same sort of tone in spoken Norwegian.
In fact this is one of the few not so easy things in Norwegian grammar. Do you know the book "Mysteriet om Nils" ("The mystery of Nils")? It contains good graphical representations of the rules for word order.
Or did you mean you can't tell the difference between main and dependent clauses? That's rather easy: dependent clauses are headed by either a conjunction (here: "hvis") or a relative pronoun.