Actually, there are 71 different isomers of C5H80.
C-CCOC=C this is e-(ethenyloxy)prop-1-ene
OCCC#C this one is pent-4-yn-1-ol
OC1CC-CC1 jc.3 or cyclopent -3-en-ol
CC(=0)CC1CC1 or 1-cyclopylathen-1-one
O=C1CCCC1 or cylopentanone
You can also break this down further:
Alcool has 3:
Bicylique
Cyclique
diene
yne
Enol
Aldehyde
cyclique
lineaine
Certonie
cyclique
lineaine
To understand isomers you must realize that an isomer a molecule that has the molecular as a different molecule. Each of these molecules has a different chemical structure. Isomers must contain the numbers of atoms for each isomer. Normally isomers won't won't always share the same properties unless they are in the same fuctional group. You can see this in Alcool, Alehyde and Cetronie.
I highly recommend an organic chemistry video with rules on how to drawn skeletal and kekule structures and isomerism (as a full course) if someone wants to understand and practice.