Majid, if you rightclick on a node you get a choice; select internal and you're inside that node. usually there's nothing there (unless somebody else put something in), but you can create another node there, or paste one from a copied outside node. Give it an appropriate name (if you have a lot of nodes, put an A in front, so it's at the top of the list). Then go a level up, go to the node again (which now has an internal node, and hover above the input that you need it to connect to (displacement, child, mask, whatever), rightclick and choose the node you're working on, but the one with the network sign in front, click that, then you see the nodes that are inside. click the appropriate one, and it's connected. You see the input triangle disappear.
I hope this makes sense. Once you get the hang of it, it's very easy.
