1. The problem asks to write the number $4.58 \times 10^{-3}$ as an ordinary decimal number. 2. The scientific notation $a \times 10^{n}$ means move the decimal point in $a$ by $n$ places. 3. Since the exponent is $-3$, we move the decimal point 3 places to the left. 4. Starting with $4.58$, moving the decimal 3 places left gives $0.00458$. 5. Therefore, $4.58 \times 10^{-3} = 0.00458$. This is the ordinary decimal form of the given number.