  In this article we will develop some techniques aimed at the strong couplings
in two-dimensional wave-Klein-Gordon system. We distinguish the roles of
different type of decay factors and develop a method which permits us to
"exchange" one type of decay into the other. Then a global existence result of
a model problem is established. We also give a sketch of the
Klein-Gordon-Zakharov system and establish the associate global existence
result.
