The first picture is when I extruded after selecting two vertices and it gave me an additional face, three edges and two vertices as you can see. But the second picture is when I extruded after selecting (some)two vertices and it gave me no faces and only additional two edges.
It seems like if I select one already extruded vertex and one that I didn't, the second scenario happens. Why? And how do I make a new face in the latter case?