Examples with animals:

Easy case:

Cat Kissa Kissat
Dog Koira Koirat
Fish Kala Kalat
Bear Karhu Karhut
Cow Lehmä Lehmät

Many times last i changes to e

Mouse Hiiri Hiiret
Moose Hirvi Hirvet


KPT rule

Pig Sika Siat

k disappears

Crap Rapu Ravut

p changes to v

Worm Mato Madot

t changes to d


If two K, P or T then other disappears

Rat Rotta Rotat
Rooster Kukko Kukot