I have a shape, which I designed in Inkscape. I uploaded back into Inkscape as PNG or SVG (tried both), and I want to add a line into the shape, which my cutting machine will cut. I want a cut in the paper to hold an end of a thread. I have tried combining then path/cut, just path/cut, then adding a rectangle on top then combine/path/cut, etc.... Do I need to make the shape over? Then how do I add the cut into the shape? Thanks for any help!
We do not know how your cutting software works and what it demands. If it cuts along SVG paths you can make one which starts inside your shape.
A closed path. Node A is selected and the curve is splitted to open path by breaking it with the node tool at node A. After the splitting both the start and end node of the path are at the same place.
One of the nodes is selected with the node tool and moved temporarily downwards 5 ticks (set in preferences how much a tick means) with an arrow key
The other node is selected with the node tool and then the curve is continued upwards with the pen tool (=2 left clicks and one right click to stop at B)
The temporarily moved node is moved back 5 ticks
I guess it's useful to try to program the cutting to start from B.
ADD after the questioner revealed he uses Cricut cutting machine:
Without any special settings Cricut's control software obviously thinks the inserted piece AB is unwanted garbage and removed it automatically. Are such settings possible? That stays hidden for non-customers.
All shown design examples seemed to have closed outlines. In Inkscape you can subtract another closed shape from your actual design shape to make a notch:
The red shape is drawn on the blue one, both are selected and Path > Subtract is applied.
It's unknown how narrow the red piece can be before the result is considered to be garbage.
A tutorial video had a hint: Use scissors or a blade for easy cuts.