How would you find all the isomers of the formula C4H8O2 (ester)?

There are four isomeric esters with the formula ##"C"_4"H"_8"O"_2##.

First, draw all the esters of a one-carbon acid. These are

Propyl formate

##"HCOOCH"_2"CH"_2"CH"_3##

and

Isopropyl formate

##"HCOOCH"("CH"_3)_2##

Then draw all the esters of a two-carbon acid:

Ethyl acetate

##"CH"_3"COOCH"_2"CH"_3##

Finally, draw all the esters of a three-carbon acid:

Methyl propionate

##"CH"_3"CH"_2"COOCH"_3##

And that's it!