Biernik czy narzędnik (Accusative or Instrumental)

Thanks strzyga. That's a concise answer. Just as long as I wasn't missing anything here.

Dzienki :-)

On second thought, there seems to be a pattern for these irregularities:
przyjaciel - przyjaciele - przyjaciółmi (not przyjacielami)
ludzie - ludźmi (not ludziami)
koń - konie - końmi (not koniami)
dłoń - dłonie - dłońmi (not dłoniami)

but then, you've got: słoń - słonie - słoniami (not słońmi!)

...so I give up. It's worse than sudoku, especially after all that beer which I bought for the Poland-England game.
Hope you have good memory, nothing else can save you :)

.......gośćMI (not: "gościami"), dziećMI (not: "dzieciami") itd....

good examples, and again, there are palatalized sounds here, so they're probably the culprits.

As posted prior, the transition from palatalized to non-palatalized (or vice-versa!) depending on the case, still causes some confusion for me:-)