problem with creating lowpoly mesh

hi guys . just a noob question i always had . and i feel too brave to ask it here in eat3d .

sry for bad  language .

when i create lowpoly mesh , lets say i have some straight faces , for getting rid of n-gons i have to add some edges in that faces and that results a weird problem .

as u can see in picture , all the vertices are aligned , but u can see the lines . and thats because of vertex normals i guess . the vertex normals are heading different ways which cause to this . if im doing something wrong please tell me or if there is a solution for this