  We investigate $S=-1$ hyperon production from the $\Lambda_c^+\to K^-p\pi^+$
and $\Lambda_c^+\to K^0_Sp\pi^0$ decays within the effective Lagrangian
approach. We consider the $\Sigma/\Lambda$ ground states, $\Lambda(1520)$,
$\Lambda(1670)(J^p=1/2^-)$, $\Lambda(1890)(J^p=3/2^+)$; $\Lambda/\Sigma$-pole
contributions from the combined resonances between 1800 MeV and 2100 MeV; and
$N/\Delta$-pole and $K^\ast$-pole contributions, which include the proton,
$\Delta(1232)$, and $K(892)$. We calculate the Dalitz plot density
$(d^2\Gamma/dM_{K^-p}dM_{K^-\pi^+}$) for the $\Lambda_c^+\to K^-p\pi^+$ decay.
The calculated result is in good agreement with experimental data from the
Belle Collaboration. Using the parameters from the fit, we present the Dalitz
plot density for the $\Lambda_c^+\to K^0_Sp\pi^0$ decay. In our calculation, a
sharp peak-like structure near 1665 MeV is predicted in the $\Lambda_c^+\to
K^-p\pi^+$ decay because of the interference effects between the
$\Lambda(1670)$ resonance and $\eta$-$\Lambda$ loop channels. We also
demonstrate that we can access direct information regarding the weak couplings
of $\Lambda(1670)$ and $\Sigma(1670)$ from the $\Lambda_c^+\to K^0_Sp\pi^0$
decay. Finally, a possible interpretation for the 1665 MeV structure beyond our
prediction is briefly discussed.
