and this is in edit mode. My face is straight right there but in rendered something goes wrong. Whats happening can you help me ?
Your face has one straight end and one that is angled, so it is not planar. The lines that you see dividing the face into triangles is an OpenGL artifact as it tries to make sense of the face. For this to look appropriate you would have to divide this face up into triangles, but the edge that you add would create a seam.
Then what i can do? Will it affect too much ?