To expand the expression \(- (x - y)(x^2 + xy - 1)\), follow these steps:

1. Distribute \(-(x - y)\) across each term in the polynomial \(x^2 + xy - 1\):

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

2. Now expand each term:

- For \(-(x - y)x^2\):
\[
-x^3 + yx^2
\]

- For \(-(x - y)xy\):
\[
-x^2y + y^2x
\]

- For \((x - y)\):
\[
x - y
\]

3. Combine all the terms together:
\[
-x^3 + yx^2 - x^2y + y^2x + x - y
\]

4. Rearranging and combining like terms gives:
\[
-x^3 + (y - x^2)y + (y^2 + 1)x - y
\]

Putting it all together, the expanded form is:
\[
-x^3 + (y - x^2)y + (y^2 + 1)x - y
\]

This is your final result for the expansion.