I noticed in my last 2 blender files, that when i use subdivision surface, unwanted lines appear in quick rendering what is this? how can i fix that?
The problem is only present in orthographic view. Changing to perspective or a camera view will fix this problem.
It's also why the issue isn't present in the final render, because most of the time you will be rendering with a perspective camera. You can actually zoom into any mesh in orthographic view and small cracks will start to appear on the mesh.
It is a known issue, with the known issue page stating:
When using Instancing, Motion Blur or Dynamic BVH, self-intersection may occur due to precision issues. Switching to Static BVH may avoid some problems but not all