Here are some practices that you may find helpful when working with matrices in Sage.

Break the matrix entries across lines, one for each row, for better readability by pressing Enter between rows.
A = matrix(2, 4, [ 1, 2, 1, 0, 3, 0, 4, 3 ])

Print your original matrix to check that you have entered it correctly. You may want to also print a dividing line to separate matrices.
A = matrix(2, 2, [ 1, 2, 2, 2]) print A print "" A.rref()