Plurality

Most masculine and neutral words (words that end with o) are replaced with an i as the last letter. For example: sagitto "arrow" > sagitti "arrows" caballo "horse" > caballi "horse"

Most, if not all feminine word (words that end with a) are replaced with an e as the last letter. For example: sora "sister" > sore "sisters" matra "mother" > matre "mothers"

Irregular Plurality
The pluralization of words that end with atio are irregular, the plural form would be ationi. For example: creatione "creation" > creationes "creations" dominatione "domination" > dominationes "dominations"

The pluralization of words that end with tor are irregular. Even though it lacks an o, it is still masculine, so it is pluralized with an i. For example: pascator "shepherd" > pascatori "shepherds" mactator "killer" > mactatori "killers"

The pluralization of words that end in ario are not as irregular, but could still be challenging. All words that end with ario are pluralized to be arii. For example: aquario "water carrier" > aquarii "water carriers".