What is the IUPAC name of CH 3 CH CH 2 CH 3 CHO ?

The given compound is:

$\underset{4}{\mathrm{C}}{\mathrm{H}}_3\underset{3}{\mathrm{C}}{\mathrm{H}}_2\underset{2}{\mathrm{C}}\mathrm{H}\left({\mathrm{CH}}_3\right)\underset{1}{\mathrm{C}}\mathrm{HO}$

There are four carbon in the longest chain, hence, the root word is but. All carbon in the longest chain are connected with single bonds, hence, an is the primary suffix. Aldehyde is the major functional group, hence, al is the secondary suffix. Methyl substituent is attached to the second carbon so $2-$methyl will be used as prefix. 

Hence, by combing all of these things $\mathrm{IUPAC}$ name of this compound is $2-$methylbutanal.