  Based on the predictions of the relevant form factors from the covariant
light-front quark model, we show the branching fractions for the $D (D_s) \to
(P,\,S,\,V,\,A)\,\ell\nu_\ell$ ($\ell=e$ or $\mu$) decays, where $P$ denotes
the pseudoscalar meson, $S$ the scalar meson with a mass above 1 GeV, $V$ the
vector meson and $A$ the axial-vector one. Comparison with the available
experimental results are made, and we find an excellent agreement. The
predictions for other decay modes can be tested in a charm factory, e.g., the
BESIII detector. The future measurements will definitely further enrich our
knowledge on the hadronic transition form factor as well as the inner structure
of the even-parity mesons ($S$ and $A$).
