  The shell-model investigation is performed to show the impact on the
structure of $^{14}$C contributed by the off-diagonal cross-shell interaction,
$\langle pp \mid$V$\mid sdsd\rangle$, which represents the mixing between the
$0$ and $2\hbar\omega$ configurations in the $psd$ model space. Observed levels
of the positive states in $^{14}$C can be nicely described in $0-4\hbar\omega$
or a larger model space through the well defined Hamiltonians, YSOX and WBP,
with a reduction on the strength of the $\langle pp \mid$V$\mid sdsd\rangle$
interaction in the latter. The observed B(GT) values for $^{14}$C can be
generally described by YSOX, while WBP and their modifications on the $\langle
pp \mid$V$\mid sdsd\rangle$ interaction fail for some values. Further
investigation shows the effect of such interaction on the configuration mixing
and occupancy. The present work shows examples on how the off-diagonal
cross-shell interaction strongly drives the nuclear structure.
