Most Latin words have several forms, depending on how the word is used in a sentence.

As a subject, "pax", as in the Christmas message of the angels, "et in terra pax hominibus" (and on earth, peace to people).

As a direct object, "pacem", as in the old canon or "round" song, "dona nobis pacem" (give us peace).

As a prepositional object, "pace", as in obituaries and on tombstones, "requiescat in pace" (may she/he rest in peace).

--Loquamur, Ph.D. in ancient Greek, college professor of Greek, Latin, German, French, and Spanish