verum ego hoc quod iam pridem factum esse oportuit certa de causa nondum adducor ut faciam.
Cicero is explaining why he can't kill Catiline just yet, and I think that "hoc" refers to the execution of Catiline.
My textbook (but this isn't for a course I am taking) tells me that
1. the main clause is: "verum ego hoc ... certa de causa nondum adducor..."
2. the relative clause is: "quod iam pridem factum esse oportuit"
3. the result clause is: "ut faciam"
My translation so far is
But I am prompted from a specific cause not yet to do THIS, which ought to have been done already long ago.
As you can see, my "hoc" (which in my translation is the capitalized "THIS") is part of the so-called "result clause"--at any rate, I can't see how it fits into the main clause. But I bet the book is right, since the "hoc" is so very far away from the result clause. So how on earth can "hoc" fit into the main clause? Could it possibly be ablative?