1Forming the plural of nouns adopted from Latin ending in -us.‘foci’‘timpani’
2Forming the plural of nouns adopted from Italian ending in -e or -o.‘dilettanti’
The suffix -i is one of several suffixes in the English language that form foreign plurals (-a and -ae are two others). Many nouns derived from a foreign language retain their foreign plural, at least when they first enter English, and particularly if they belong to a specialist field. Over time, though, it is quite normal for a word in general use to acquire a regular English plural. This may coexist with the foreign plural (e.g., cactus, plural cacti or cactuses), or it may actually oust a foreign plural. Note that not all Latin words ending in -us have a plural that ends in -i: for example, the Latin word apparatus is unchanged in the plural.
Forming adjectives from names of countries or regions in the Near or Middle East.‘Azerbaijani’‘Pakistani’
From Semitic and Indo-Iranian adjectival endings.