How to understand another's program?

From: Kurda Yon (kurdayon_at_yahoo.com)
Date: 08/18/04 

Date: 18 Aug 2004 06:27:51 -0700

Hi,

can you tell me if there are same technic which can help to understand
a big program written in Fortran by another person?

Can one say how difficult is it? Is there some critical size of
program, after which it is almost impossible to restore a way of
thinking of person who created program?