As a beginner I can see how to create squares, circles and stars, but how do you create a triangle and then edit it by truncating it? 

There are several different approaches, but the most common are:

• Use the Stars & Polygons tool with the number of corners set to 3 (good for equilateral triangles).
• Draw them using the Bezier tool. Click for the three corners, then double-click on the first one to close the shape.
• Create them from another shape. I often draw a square or rectangle, convert it to a path (Path > Convert to Path), switch to the node editor (F2, or double-click on the rectangle), select one node then press Ctrl-Del to remove it whilst leaving the other nodes connected with a straight line.
• Another way to create from another shape is to use Boolean operations. E.g. draw a square and rotate it by 45° (hold Ctrl whilst rotating to make this easier). Draw another rectangle that crosses it to leave a triangular corner sticking out. Select both then use Path > Difference to cut away the area of the square that's covered by the rectangle. it's easier to do than to describe, and is good for isosceles triangles.

Triangles are just closed paths with only three nodes. Getting familiar with node editing and Boolean operations in general will make it easier to tweak and modify them once they're created.