For the first expression, simplify:

x² - y² + x + y

For the second expression, it appears to be incorrect:

5m² + 20mn + 20n²

For the third expression, expand using the formula a³ - 3a²b:

(x³) - 3(x²)(y) + 3(x)(y²) - (y³)
x³ - 3x²y + 3xy² - y³

For the fourth expression, also correct the given expression:
(a - b)³ - 3(a - b)²

For the fifth expression, simplify:

(m + n)² - n(m + n)²

These are the simplified versions of the given expressions. If there was a mistake in transcribing the expressions, kindly provide the correct ones for further simplification.