Is c.g rear or fore of wing leading edge?
For longitudinal stability in a conventional wing-and-tail layout, the CG needs to be ahead of the wing's AC.  This causes a nose-down moment that is countered by downward lift at the stabilizer, so that the stabilizer will raise the nose as speed builds up.  Thus, if the aircraft is disturbed into a dive, the nose will automatically rise as speed builds; conversely, if disturbed nose up, as speed falls off the nose will tend to drop.

Aircraft with active control (especially computer control), as well as some alternative layouts (such as a classic canard) can alter this conventional stability setup -- for instance, in a canard, the stabilizer at the nose lifts, and the CG will be ahead of the leading edge of the wing in most cases.