Te lo diré cuando nos veamos. –I`ll tell you when I see you. The main article is that I can`t leave. Why can`t I leave? Because I don`t have any money. The idea here is not that I don`t want to leave, but the fact that I can`t leave because I don`t have any money. The main article is, I`ll tell you. When will I tell you? When I see you. « I will tell you » is therefore incomplete without the juxtaposition « when I see you ». Note that many of these conjunctions must be followed by the subjunctive. Como no tengo dinero, no puedo ir. « Since I don`t have any money, I can`t leave. The main article is what I said.
What did I say? That I`m busy. « I`m busy » is incomplete without « I said. » I may not be busy, but I said it was me. Subordinate conjunctions link dependent (child) clauses to main clauses.