The essential aspects of this subject for JAR ATPL(A) exam purposes are summarised below.
If the thrust lines of the engines do not pass through the Centre of Gravity of the aircraft, any change in thrust will cause the aircraft to rotate about its C of G. If for example the thrust lines are below the Centre of Gravity the aircraft will pitch nose up each time the thrust is increased. In some aircraft the engines are fitted to the rear fuselage, so that their thrust lines are very close to the Centre of Gravity, thereby reducing the influence of thrust changes on longitudinal control.
A second benefit of rear-mounted engines is that they provide an additional horizontal surface, protruding out into the airflow behind the Centre of Gravity of the aeroplane. This causes them to act as if they were a second tailplane. If the aircraft pitches nose up, they produce a tail up moment. If the aircraft pitches nose down they produce a tail down moment. In this way they increase the longitudinal stability of the aeroplane.
But when engines are fitted to the wings, their mass reduces wing root bending stresses in flight, and tends to damp out vibration and flutter. Fitting the engines to the rear fuselage robs the wings of these benefits and actually increases the wing root bending stresses. This means that mounting the engines on the rear fuselage requires heavier and stronger wing structures to resist the increased bending loads and to counteract the increased flutter. Rear mounted engines are also usually higher, making maintenance work more difficult