  Form factors and decay widths for $D^{\ast}\rightarrow D \gamma$ and
$D^{\ast}\rightarrow D \pi$ decays are estimated in a relativistic constituent
quark model. Relativistic corrections due to light quarks are found to be
substantial and to suppress the vector and axial vector form factors. The CLEO
experimental value of $R^0_{\gamma}\equiv \Gamma (D^{\ast 0}\rightarrow D^{0}
\gamma)/ \Gamma (D^{\ast 0}\rightarrow D^{0} \pi^{0})=0.572\pm 0.057\pm 0.081$
is used to determine the quark axial vector coupling $g_A$, which is found to
be $0.6-0.8$ for $m_u=(350-200) MeV$ correspondingly, as compared with the
chiral model result $g_{A}= 0.8-0.9$. The heavy meson-pion strong coupling $g$
is found to be $0.4-0.6$, much smaller than $g=1$ which is expected in the
large $N_{C}$ and nonrelativistic limit, but consistent with some heavy hadron
chiral theory and QCD sum rule results. The failure of nonrelativistic
treatment for the light quark in the heavy meson decays is also emphasized.
