I am interested in analysis and PDEs, especially in nonlinear wave-type equations, including wave, Klein-Gordon, and Dirac equations. I mainly focus on: 1) establishing the small global-in-time solutions to such equations for certain kinds of (quadratic) nonlinearities; 2) building global existence results and unified pointwise decay results, for the Klein-Gordon equations, which are uniform with respect to the possibly vanishing mass parameter; 3) deriving sharp asymptotics of nonlinear wave-type equations, say sharp pointwise decay, scattering, and sharp energy bound. The study is mainly motivated by the cubic Dirac model, the Einstein-Klein-Gordon model, the electroweak standard model, and the Klein-Gordon-Zakharov model from mathematical physics, which is mathematically interesting and challenging.