New from Inkscape, trying to draw pen shape over pen shape of almost exact dimensions

In the Control panel (make sure that Shapes are shown) you have the option to show/hide Shape Widgets.

Hiding them will allow you to create new shapes from corners.

Hi Phillipe! If you unselect the first triangle, you'll be able to draw a new shape on top, even if you click on top of the anchor points. You can also lock the first object to avoid selecting or editing it - you can do this by selecting the object and pressing Ctrl/Cmd+2, or by using the Layers Panel. Also, a better alternative to creating rectangles is to use the Polygon Tool, if you press up or down on the arrow keys while makign a polygon, it'll change the number of sides the polygon has. By making a polygon with 3 sides, you're effectively making a triangle 🙂  more than that, it'll create an equilateral triangle, meaning all sides will have the same lenght.

Easiest would be to lock the shape underneath. Then use Smart guides for snapping purposes.

But you could also click away from the point, hold down the mouse, press the spacebar while moving the point where you need it and then release mouse and spacebar.