How to show $S$ is a free left-module over itself of rank $n$ for any $n \in \mathbb{N}$?
If $S\simeq S\oplus S$ then $S\oplus S\simeq S\oplus S\oplus S$. Can you take it from here?
If $S\simeq S\oplus S$ then $S\oplus S\simeq S\oplus S\oplus S$. Can you take it from here?