  A crucial lemma on module theory is Nakayama's lemma \cite{AF}. In this
article, we shall investigate some forms of Nakayama's lemma in the category of
right acts over a given monoid $S$ with identity 1. More precisely, among other
things, we show that equality $AI=A$ for some proper ideal $I$ of $S$ implies
$A=\{\theta\}$, when $A$ is a finitely generated quasi-strongly faithful
$S$-act with unique zero element $\theta$ and $S$ is a monoid in which its
unique maximal right ideal $\mathfrak{M}$ is two-sided. Furthermore, as an
application of Nakayama's lemma we prove Krull intersection theorem for
$S$-acts. Finally, as a consequence, we shall see a homological classification
form of this lemma, i.e, we prove if $S$ is a commutative monoid then every
projective $S$-act is free if and only if $E(S)=\{1\}$, which $E(S)$ is the set
of all idempotents of $S$.
