When I draw a line or a picture I always get a double line under "Edit paths by nodes". Is it possible to draw a line that is simple? I don't need the possibility to have fill inside the line. I just want to decide the width of the line. It would be much easier to change the shape in a picture if it's only one line.

Hi.

It depends on the tool you are using and its settings.
The calligraphy tool produces filled paths, so use either the pencil or the pen tool.
Both of those tools come with the optional setting of a pattern.
Set the shape to "None" in the tool's controls and it will work as expected.