The fleeting a

You have already met words that seem to change shape for no reason: pas (dog) but nemam psa (I don't have a dog), otac (father) but kuća mog oca (my father's house). Nothing is irregular here. Many Serbian words have an a that sits between the last two consonants of the stem, and that a comes and goes. Serbian grammar calls it the nepostojano a — the fleeting a.

The reason lies in the stem. The real stem of pas is ps-, and Serbian does not let a word end in a cluster like ps. When nothing follows the stem, an a is slipped in to make it pronounceable (ps → pas). When an ending that starts with a vowel follows, the extra a is no longer needed and drops out (ps-a, ps-u, ps-i). Other words do end in two consonants (most, prst), which is why the fleeting a has to be learnt word by word.

1. Masculine nouns: the a is there in the nominative singular only

In a masculine noun the a is visible in the dictionary form, and disappears as soon as an ending is added — genitive, dative, accusative, instrumental, locative, and the plural.

otac also loses its t: oca, ocu, ocem. Its plural is built on a longer stem, očevi. Learn it as a word, not as a model. And momci: the change of k to c before -i is the topic of the next lesson.

2. The genitive plural brings the a back

The genitive plural (the form after mnogo, pet, nema and other quantity words) ends in a long -a. In front of that ending the fleeting a comes back — so masculine nouns look like their nominative singular again, plus -a.

otac is the exception again: its usual genitive plural is očeva (pet očeva), built on the long plural stem.

3. Feminine and neuter nouns GAIN an a in the genitive plural

Here the direction is the other way round. A feminine noun in -a or a neuter noun in -o whose stem ends in two consonants has NO a in the dictionary form: sestra, devojka, pismo. Only in the genitive plural, where the ending -a leaves the two consonants together at the end of the stem, does an a appear between them.

Keep the two directions apart. Masculine: the a is in the dictionary form and LEAVES in the other forms (pas → psa). Feminine and neuter: there is no a in the dictionary form and one ARRIVES in the genitive plural (sestra → sestara). Not every such noun does this — majka has majki, torba has torbi — so learn the frequent ones by ear.

4. Adjectives: dobar, dobra

The masculine nominative singular of an adjective has no ending either, so many adjectives show the same fleeting a there — and only there. Every other form, including feminine and neuter, has an ending and loses it.

So On je umoran, but Ona je umorna, Oni su umorni. You cannot tell from spelling alone which words have a fleeting a — star, nov never do — so learn each adjective with its feminine: umoran, umorna.