Not by origin: "πάρδαλις: the pard, whether leopard, panther, or ounce" http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0058:entry=pa/rdalis
παρδαλός/πάρδαλη/παρδαλό has acquired the meaning "dappled" in modern Greek, but that came afterwards.
How do I use ι and η ?
Like English "ee" and "ea" -- you have to memorise which word uses which spelling.
Like English children who have to learn that "clean" has ea but "green" has ee even though they rhyme exactly, Greek children have to learn where to put ι and where to put η.
(Historically, the sounds were pronounced differently - in both English and Greek - and that is why they were spelled differently. The pronunciation later changed to make them the same but the spelling was kept, which now means lots of memorisation.)