I create a path with the path tool Then I click on a node and I get two handles. Sometimes these handles are disconnected, by that I mean that if I move one the other remains stationary. However sometimes they are locked together, so when I move one the other moves as well. Sometimes this behavior is what I want, but sometimes not. So how do I switch between the two modes?

Thanks.

You can change the node type (smooth, cusp, auto,...) by clicking on the icons for the node types in the node tool's tool control bar.
Find more info here:
http://www.flossmanuals.net/inkscape/toolbox/node-tool/
http://tavmjong.free.fr/INKSCAPE/MANUAL ... iting.html

For a closed shape, the sometimes the end points of the path have handles that are disconnected. Change the node type to connect them.