Declination of feminine nouns ending in -ia

Hello,

I have a problem memorizing the rule to decline feminine nouns ending in -ia into singular dopełniacz, celownik and miejscownik. I know the declined name is supposed to end in -i but it's not clear for me when the final -a is dropped or when it is replaced with a second "i".

So, names in -ia:
In which cases do they end in "-i" ?
In which cases do they end in "-ii" ?

Thanks

The general rule of thuimb is: wrods of Latin origin get the -ii, whilst inidgenously Polish words get -i. Hence historii, armii and geografii but bani, stajni and Kasi.

For the -ii, not only of Latin, but more generally of foreign origin: Holandii, Norwegii, Danii, Bułgarii, malarii, Marii.

For the -i, please notice that nouns like szyja, nadzieja will also end in -i , droping its j in the relevant subordinate cases. Hence: szyi, nadziei.

Thanks for your explanations.