I have a cylinder and I want to cut a rectangular hole in it,I do it but when adding a subsurf modifier to the object and add some proximity loops but I fact this problem as you see here
how can I fix this problem
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You could try to select the edges of you rectangle and increase the Crease Weight in the Properties Panel N.
If this doesn't work well you could subdivide the cylinder before cutting, this way the influence of the Subsurf Modifier would be more local.