Adding up to What are the left and right ideals of matrix ring? How about the two sided ideals?. Let's say that a left ideal in $M_n{R}$ is defined as the set of matrices whose rows entries are elements in a submodule $M$ of $R^n$. (The ring is commutative with neutral element.).
Why do all other left ideals have the same form as described above?
It´s been 2 days i have been handling this problem.
