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!