The Fleeting 'a' (nepostojano a)

The first time you meet pas ("dog") and then psa ("of the dog"), it looks as if the noun has been mangled — where did the a go? It did not get deleted by some irregular rule; it was a fleeting a (nepostojano a), a vowel that exists only to make the citation form pronounceable and disappears the moment an ending gives the cluster something else to lean on. The same phenomenon, running in reverse, inserts an a into the genitive plural of many feminine and neuter nouns (sestra → sestara). Learners almost always meet these as two unrelated "irregularities." They are one phenomenon — a single vowel that comes and goes to keep consonant clusters speakable — and seeing them together is what turns a long list of exceptions into one tidy rule.

What the fleeting a does

Croatian dislikes certain word-final and stem-final consonant clusters. The fleeting a is the language's repair tool: it sits before the final consonant of a form, breaking a cluster that would otherwise be hard to say. It appears in exactly two places:

1. The nominative singular of many masculine nouns (and some adjectives), where the bare stem would end in an ugly cluster: pas (not ps), otac (not otc), vjetar (not vjetr).
2. The genitive plural of many feminine -a and neuter nouns, where dropping the ending exposes a cluster: sestara (not sestr), pisama (not pism).

The unifying logic: the fleeting a shows up precisely where there is no vowel ending to do the job, and vanishes the instant an ending arrives to support the cluster. In pas, there is no ending, so the a props up ps; in psa, the genitive -a ending now follows the cluster, so the propping a is no longer needed and drops.

Masculine nouns: the a drops

The classic pattern. The a is there in the nominative singular and the accusative inanimate (which equals it), but it drops in every oblique case because those cases add a vowel ending.

Notice that the accusative of vjetar and lonac (inanimate) keeps the a — because the inanimate accusative equals the nominative, the form with no ending. But the accusative of pas and otac (animate) equals the genitive psa, oca, so the a drops there too. The fleeting a tracks the ending, not the case label.

The -ac and -ak suffixes are the engine

A huge share of fleeting-a masculines end in the suffixes -ac and -ak (and the -ar/- er type like vjetar, metar). Words formed with these suffixes — borac ("fighter"), lovac ("hunter"), stranac ("foreigner"), momak ("young man"), početak ("beginning") — all drop the a in oblique cases. Once you recognise the suffix, the behaviour is predictable.

A bonus complication: voicing

In a few words, dropping the a brings two consonants into contact that then assimilate in voicing. The textbook case is vrabac ("sparrow"): the genitive is vrapca, not vrabca — the b devoices to p before the voiceless c. So the spelling itself changes, not just the vowel.

This double change is treated under voicing assimilation; just know that the fleeting a can trigger it.

Adjectives do it too

The same vowel lives in many masculine adjective forms. The masculine nominative singular (indefinite) carries the a; the feminine, neuter and all longer forms drop it: bolestan ("sick," masc.) → bolesna (fem.), bolesno (neut.), bolesni (def.). Likewise dobar → dobra, gladan → gladna, pametan → pametna.

The reverse: the a is INSERTED in the genitive plural

Now the other half of the same coin. Many feminine -a and neuter nouns form their genitive plural with a zero ending (the noun is left bare), which would expose the cluster at the end of the stem. So Croatian inserts a fleeting a before the final stem consonant to break it: sestr- → sestara, pism- → pisama, djevojk- → djevojaka.

*The masculine momak is special: it drops the a in the singular oblique (momka) AND re-inserts one in the genitive plural (momaka) — the same vowel doing both jobs in one word, which is the clearest possible proof that drop and insertion are a single phenomenon.

The decision is purely phonetic: insert only if the bare stem ends in a cluster you would not want to pronounce unsupported. Žena and knjiga end in a single consonant + (the dropped) vowel, so nothing is inserted — žena, knjiga stand as they are. Sestra ends in -str-, which is unspeakable bare, so sestara.

How this differs from English

English has nothing like this. The closest analogy is the way English inserts a vowel sound in plurals like bushes or in the past tense wanted — a "linking" vowel to make a cluster pronounceable — but English never removes a written vowel as a grammatical consequence of adding an ending, and it certainly does not do it inside the stem. For an English speaker, pas → psa feels like the word has been corrupted. Reframe it: the a in pas was never part of the root; it was scaffolding, present only because the bare root ps could not stand alone. Add an ending and the scaffolding comes down.

Common Mistakes

Key Takeaways

• The fleeting a (nepostojano a) is one phenomenon, not two: a vowel that drops in oblique forms (pas → psa) and inserts in the genitive plural (sestra → sestara).
• It appears wherever a cluster would be unsupported (no vowel ending) and vanishes when an ending follows.
• The -ac / -ak / -ar suffixes are the main masculine engine (borac → borca, momak → momka, vjetar → vjetra); adjectives do it too (bolestan → bolesna).
• Dropping the a can trigger voicing assimilation: vrabac → vrapca.
• In the genitive plural, insert the a only when the bare stem ends in a cluster (sestara, pisama, djevojaka) — not when it is already speakable (žena, knjiga).