Let’s say i add a plane, it has 4 vertices. What if I want to add one vertice
in the middle of one edge, can I do that? With the knife tool you can but it
will make two edges to the opposite vertices. I want to know if I can add
one vertice, nothing else.
digiman:confused:
Go into vertex select mode CTRL+LMB and you will get one vertex, nothing else. 
You can’t have five sided polygons (n-gons) in blender.
Wait !
CTRL-LMB will add a new vertex but it won’t be conneced to the middle of the Edge of the plane - if that’s what your looking for then it’s OK !
Salutations
As N30N points out, you can’t have five sided faces in blender. When you use the knife tool, Blender makes the new vertex, but then redraws the face using three triangles. The best you can do with a single plane, is select two of the tris and join them, leaving you with a quad and a tri.
In the middle !
W-Key and divide the edge as you wish
but it will add other edges too - That’s life
Salutations