If $p$, then $q$. Not $q$. Therefore, not $p$. If all prime numbers are odd, then 2 is odd. 2 is not odd. Therefore, it is not the case that all prime numbers are odd.
Work Step by Step
Replace each complete idea (i.e., an idea that can be expressed as a complete, independent sentence) as a single letter (here, $p$ or $q$). Fill in the blanks in the example argument based on where you put the letters $p$ and $q$ by examining the first argument.