  In this work we consider the problem of global existence of small regular
solutions to a type nonlinear wave-Klein-Gordon system with semi-linear
interactions in two spatial dimension. We develop some new techniques on both
wave equations and Klein-Gordon equations in order to get sufficient decay
rates when energies are not uniformly bounded. These techniques are compatible
with those introduced in previous work on two spatial-dimensional quasi-linear
wave-Klein-Gordon systems, and can be applied in much general cases.
