Pies to Psa, but why not Piesa?

I’m sure you’re aware that basically as you change the case of a noun, you often need to change the ending of the word in some way.

There are dozens of rules about when and why (which I’ll have a go at simplifying some of in the future), but one example you see again and again is Pies (Dog), but it’s a strange one because in various cases Pies turns into Psa, Psy, Psu or Psów.  Yet Kot (Cat) (which is another masculine animate noun) changes to Kota, Koty, Kotu, Kotów.

There are other words which do similar things, where some letters are removed before the ending is applied, but until today I hadn’t found anywhere that explained why these words were special, so I had them all lumped together as special nouns you had to memorize, but I really don’t like having to memorize more than necessary.

Today I found a book which gave me a rule to follow, which appears to hold true for lots of other nouns, so I wanted to share while it was fresh in my mind.

If the noun has an ‘e or an ‘ie in the last syllable, then remove it before adding the ending.

Examples of this pattern (with just the one other case to keep it simple)

Nominative Accusative English
Lew Lwa Lion
Dziadek Dziadka Grandfather
Chłopiec Chłopca Boy
Ojciec Ojca Father

Note this only applies (as far as I can tell) to Masculine Animate nouns, but at least it’s one more pattern and one less magic special case to remember.