  We investigate the transition form factors $f_+(q^2)$ and $f_-(q^2)$ [or
$f_0(q^2)$] for the exclusive semileptonic $B\to D\ell\nu_\ell$ $(\ell=e,
\mu,\tau)$ decays in the standard light-front quark model (LFQM) based on the
LF quantization. The common belief is that while $f_+(q^2)$ can be obtained
without involving any treacherous contributions such as the zero mode and the
instantaneous contribution, $f_-(q^2)$ receives those treacherous contributions
since it involves at least two components of the current, e.g. $(J^+, J^-)$ or
$(J^+, {\bf J}_\perp)$. Contrary to the common belief, we show in the Drell-Yan
($q^+=0$) frame that $f_-(q^2)$ obtained from $(J^+, J^-)$ gives identical
result to $f_-(q^2)$ obtained from $(J^+, {\bf J}_\perp)$ without involving
such treacherous contributions in the standard LFQM. In our numerical
calculations, we obtain the form factors and branching ratios for $B\to
D\ell\nu_\ell$ $(\ell=e, \mu,\tau)$ and compare with the experimental data as
well as other theoretical model predictions. Our results for ${\rm Br}(B\to
D\ell\nu_\ell)$ show reasonable agreement with the experimental data except for
the semitauonic $B^0\to D^-\tau\nu_\tau$ decay. The ratio ${\cal
R}(D)=\frac{{\rm Br}(B\to D\tau\nu_\tau)}{{\rm Br}(B\to D\ell'\nu_{\ell'})}$
$(\ell'=e,\mu)$ is also estimated and compared with the experimental data as
well as other theoretical predictions.
