Write the structure of 3-methylbutanal.

First draw the main chain of 4 carbon atoms, since there is ‘but’.

Next, we put the –CHO group at one end and from that end we start numbering.

Next, we put a –CH₃ group on the 3^rd carbon atom as the name says 3-methylbutanal.

The final step is to fill the carbon atoms with required H atoms.