Using $20.3\,\mathrm{fb}^{-1}$ and $7.33\,\mathrm{fb}^{-1}$ of $e^+e^-$ collision data collected at the center-of-mass energy of 3.773 and $4.128-4.226$ GeV with the BESIII detector, the non-perturbative nature of QCD in the charm sector is investigated.
The decays of $D^+\to f_0(500) \mu^+\nu_\mu$, $D\to b_1(1235) e^+\nu_e$, $D\to K_1(1270) \mu^+\nu_\mu$ and $D^+\to K_S^0\pi^0\mu^+\nu_\mu$ are observed for the first time. Based on the amplitude analyses of $D_{(s)}\to hh \ell^+ \nu_\ell$ and $hhh \ell^+ \nu_\ell$ processes, the scalar($a_0(980)$, $f_0(500)$ and $f_0(980)$), vector($K^*(892)$, $\phi(1020)$, $\rho(770)$), axial-vector($K_1(1270)$, $b_1(1385)$) particles are studied. The measurements of $D \to a^0(980)$, $D\to f_0(500)$, $D\to K^*(892)$, $D\to \rho(770)$, and $D_s \to \phi(1020)$ form factors are presented.