Subtract the expression $m{n}^2+m^{2}n-mn$ from the expression $m^{2}n-n^2-mn$.

$\left(m^{2}n-n^2-mn\right)-\left(m{n}^2+m^{2}n-mn\right)$

$=m^{2}n-n^2-mn-m{n}^2-m^{2}n+mn$

Collecting the like terms together, we get

$=m^{2}n-m^{2}n-mn+mn-n^2-m{n}^2$

$=-n^2-m{n}^2$