Merging and re-splitting cells

I and many of my fellow users are often bemused by the behaviour of
cells when, for example, a cell from resulting merged cells is later
split again. In some cases the cell boundaries match the boundaries of
un-merged cells in earlier rows or columns, and in some cases they
don't. And how do I tell if a cell boundary very close to a boundary
above or to its left is the same as its predecessor's boundary (and
will move with it)? And is the behaviour for rows and columns the same
(I don't think so). And why am I sometimes obliged to split a cell
using a divisor ot the number of cells from which it was merged, and
sometimes not?

An short explanation (if it's possible!) of the structures underlying
the organisation of a table would greatly help to explain and avoid
difficulties in table layout. Can anyone help?