The so-called “fleeting A” or “movable A” refers to the phonetic alternation in which short a makes an appearance and loss in certain inflected forms of nouns.
Historically, Montenegrin did not tolerate any final consonantal clusters except st, zd, št, žd, šć, žđ, šč, and dž, as they were often difficult to pronounce in the final position. Thus, in order to facilitate pronunciation a fleeting a was inserted. The following suffixes are especially prone to the rule – ac, ak, an, ar, at, anj, alj, ao, ac.
In Nominative, Accusative, and Genitive singular and Genitive plural of masculine nouns:
A writer – pisac >pisca > pisaca
A young man – momak > momka > momaka
A dream – san > sna > snova
In Genitive plural forms of feminine nouns ending in a consonant cluster:
A sister – sestra > sestara
A notebook – sveska > svesaka
In Nominative singular indefinite masculine forms of adjectives and pronouns:
Short – kratak > kratki
Entire – sav > svi