We characterize prime submodules of $R\times R$ for a principal ideal domain $R$ and investigate the primary decomposition of any submodule into primary submodules of $R\times R.$.
In this paper we characterize all prime and primary submodules of the free $R$-module $R^{n}$ for a principal ideal domain $R$ and find the minimal primary decomposition of any submodule of $R^{n}$. In the case $n=2$, we also determine the height of prime submodules.