Step 1: Identify the target transformation.
Ethanal ($CH_3CHO$, an aldehyde with 2 carbons) must be converted to ethane ($CH_3CH_3$), which requires complete removal of the oxygen from the carbonyl group and replacement by two hydrogen atoms.
Step 2: Why simple reduction is insufficient.
Mild reducing agents like $NaBH_4$ reduce the aldehyde only to the primary alcohol (ethanol), not to the alkane. A more forceful method is required to convert the $-CHO$ directly to $-CH_3$.
Step 3: Clemmensen reduction.
Heating ethanal with zinc amalgam (Zn-Hg) and concentrated hydrochloric acid under reflux (Clemmensen reduction) directly reduces the carbonyl group to a methylene group ($-CH_2$-), giving the alkane.
Step 4: Write the equation and note the alternative.
$CH_3CHO \xrightarrow{Zn\text{-}Hg / \text{conc. } HCl, \Delta} CH_3CH_3 + H_2O$. An equivalent method in basic conditions is Wolff-Kishner reduction using hydrazine and $KOH/\Delta$.