You meet the sentence Nemam psa. (“I don’t have a dog”), open a dictionary to look up psa, and find nothing. Nothing under ps-, nothing nearby. The word is there, but in another shape: pas (“dog”). Between pas and psa a whole vowel has vanished. This is the fleeting a (in Serbian nepostojano a, “the unstable a”): a small letter that appears in one form of a word and disappears in another. It trips learners up in the dictionary, in declension and in word building. Yet its logic is simple, and English has a quiet version of the same habit. Below: where it lives, why it comes and goes, and how to spot it.
Two sides of one coin: in and out
Pairs of forms show at once how the a behaves: the dictionary form on the left, a form with an ending on the right. Most right-hand forms are the genitive, the “of” case you have met after nemam (“I don't have”); it gets its own lessons later.
Notice the pattern. While the word ends in a consonant (pas, momak), the a keeps the last two consonants apart so the tongue does not stumble. Attach an ending that begins with a vowel (-a, -u, -om), and that vowel takes over the job of separating them; the fleeting a becomes redundant and drops out: momak, but momka, momku, momkom. (In otac → oca there is one more small change: the t that would now stand right before c is swallowed; the lesson “Cluster changes” comes back to it.)
Where the fleeting a lives. Home one: masculine nouns
The main home of the fleeting a is the nominative singular of masculine nouns, the dictionary form. Many masculine nouns hold an a in the last syllable of that form and lose it in the other cases. Most often these are words in -ak and -ac (momak, petak, novac, borac), less often in -ar and -an (vetar, san). The ending alone proves nothing, though: in lekar → lekara (“doctor”) and stan → stana (“flat”) the a is stable. And one twist for the fleeting ones: in the genitive plural many of them get their a back, as in pet momaka (“five young men”) and pet pasa (“five dogs”).
Look at vrabac → vrapca: here the a not only dropped out but also set off a second change, b turned into p. That is a different law, and the next lesson, “Voicing assimilation”, is about it. For now just keep in mind that once the a is gone, two consonants meet head-on and may adjust to each other. Where the a comes back, so does the b: vrabaca (“of sparrows”).
Cheat sheet
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The fleeting a (nepostojano a) is a vowel that stands in one form of a word and drops out of another, between the last two consonants of the stem (the word minus its ending: dobr-). In one line: the a breaks up a consonant cluster, and when an ending’s vowel moves the consonants apart, it disappears. English knows the idea: hunger → hungry.
Home one: masculine nouns, nominative singular
Commonest in -ak, -ac, less so in -ar, -an; but lekar → lekara, stan → stana keep it. A vowel ending (-a, -u, -om) separates the consonants, so the a drops out. In the genitive plural many get it back: pet momaka, pet pasa. In vrabac → vrapca the loss also triggers voicing assimilation: b becomes p.
Home two: masculine adjectives
Home three: genitive plural, where the a pops up
Here the a eases a consonant cluster before the ending (pet sestara, “five sisters”).
Not every a is fleeting
In grad, brat, sat the a belongs to the stem and stays in every form (grada). In momak the stem is momk-: the a only breaks up the final cluster and drops out once an ending is added. Likewise, in vrabac the second a runs away, the first belongs to the root; in Beograd the only a is stable (Beograda).
Two frequent words
jedan (“one”) keeps the a only in the bare masculine form (nominative; accusative of things: Imam jedan problem): jednog, jednom, jedna (feminine), jedno (neuter).
sav (“all, the whole”): sva (feminine), sve (neuter), svega (Gen.).
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