  We study the $d\Gamma / dq^2$ spectra and the branching fractions of the $D$
meson exclusive semileptonic decays with the lepton mass effects into
consideration. We investigate their sensitivity to form factor models, and find
that the decays to a pseudoscalar meson and a lepton pair are sensitive to the
property of the form factor $F_1(q^2)$, and those to a vector meson and a
lepton pair to the form factor $A_1(q^2)$. We also analyze the experimental
results of the branching fractions $\cal{B} (D^0\to K^- (or \pi^-) \pi^+)$ and
$\cal{B} (D^0\to K^- (or \pi^-) e^+ \nu)$, and show that it is implied that
$F_1(q^2)$ is of dipole type, instead of simple pole type which is commonly
assumed.
