Modes theory plays an important role in physics as the general evolution of a system could be expressed as the superposition of its eigenmode. For closed or periodic systems, the eigenmode is normal mode with real eigenfrequency. However, nearly all realistic systems are open, where the dissipation is via radiation to the far field region and/or absorption within the system. Therefore, to describe the modes of such an open system, the quasinormal mode (QNM) theory has been developed in the last few decades, which provides deep insight and efficiency for describing electromagnetic scattering and light-matter interaction. Here, I will present a few works about the QNMs theory of Purcell factors and spontaneous emission decay rates near nano/micro resonators, including coupled QNMs theory, breakdown of usual Fermi's golden rule in the presence of linear gain, quantum/classical fix of spontaneous emission rate with linear gain, gain compensation to improve the Purcell factors of gold-dimer plasmon modes by over one million-fold, chiral emission for linear dipole emitters near a refractive index modulated microring.