We cannot simplify the square root of a negative number into a real number because the square of any real number is always positive or zero. However, we can simplify the expression -4 - √(-18) by writing it in terms of the imaginary unit 'i', which is defined as the square root of -1.
√(-18) = √(92-1) = 3i√2
Therefore, -4 - √(-18) = -4 - 3i√2
This is the simplified form of the expression.