  For an irreducible module $P$ over the Weyl algebra $\mathcal{K}_n^+$ (resp.
$\mathcal{K}_n$) and an irreducible module $M$ over the general liner Lie
algebra $\mathfrak{gl}_n$, using Shen's monomorphism, we make $P\otimes M$ into
a module over the Witt algebra $W_n^+$ (resp. over $W_n$). We obtain the
necessary and sufficient conditions for $P\otimes M$ to be an irreducible
module over $W_n^+$ (resp. $W_n$), and determine all submodules of $P\otimes M$
when it is reducible. Thus we have constructed a large family of irreducible
weight modules with many different weight supports and many irreducible
non-weight modules over $W_n^+$ and $W_n$.
