  In this paper we prove that for a monoid $S$, products of indecomposable
right $S$-acts are indecomposable if and only if $S$ contains a right zero.
Besides, we prove that subacts of indecomposable right $S$-acts are
indecomposable if and only if $S$ is left reversible. Ultimately, we prove that
the one element right $S$-act $\Theta_S$ is product flat if and only if $S$
contains a left zero.
