1. **Problem:** Simplify the expression $-y^{-3}$. 2. **Recall the rule:** Negative exponents mean reciprocal, so $a^{-n} = \frac{1}{a^n}$. 3. **Apply the rule:** $$-y^{-3} = -\frac{1}{y^3}$$ 4. **Final answer:** $$-\frac{1}{y^3}$$ This means the expression is the negative reciprocal of $y$ cubed.