\(y-x^2y-xy^2-y^2\)
\(=\left(y-x^2y\right)-\left(xy^2+y^2\right)\)
\(=y\left(1-x^2\right)-y^2\left(x+1\right)\)
\(=y\left(1-x\right)\left(x+1\right)-y^2\left(x+1\right)\)
\(=\left(x+1\right)\left[y\left(1-x\right)-y^2\right]\)
\(=\left(x+1\right)\left(y-xy-y^2\right)\)
\(=\left(x+1\right)\left[y-\left(xy+y^2\right)\right]\)
\(=y\left(x+1\right)\left(1-x-y\right)\)