Piece of cake! Take your cube and rotate the view with MMB so that you can see all verts. Then on one side of the cube select two verts opposite of each other on the diagonal and Fkey to draw an edge between them. Do the same for the other side of the cube. Then select the two verts of the part of the cube you don’t want and Xkey them. Voila! A cube cut in half!
There are really dozen of ways to create roof geometry. Here is one way (Jagguy2 beat me to it).
Make a rectangle (main shape of house). Extrude top face, select top two vertices (image 1 and circle around the vertices), and merge those points at their center. Do that for the back too to make the simple roofline.
As for making an edge, what you do is to select a vertex then hit “E” key to extrude that vertex. New vertex forms and as you drag the vertex new edge is created.