Subtract $xy$ from $ \dfrac{x}{y^3} $ to get $ \dfrac{ \color{purple}{ -xy^4+x } }{ y^3 }$.

Step 1: Write $ xy $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{y^3}$.

$$ \begin{aligned} \frac{x}{y^3} -xy & \xlongequal{\text{Step 1}} \frac{x}{y^3} - \frac{xy}{\color{red}{1}} = \frac{ x }{ y^3 } - \frac{ xy \cdot \color{blue}{ y^3 }}{ 1 \cdot \color{blue}{ y^3 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ x } }{ y^3 } - \frac{ \color{purple}{ xy^4 } }{ y^3 }=\frac{ \color{purple}{ -xy^4+x } }{ y^3 } \end{aligned} $$