Reconstruction:Proto-Slavic/vъ(n)

Either:

• From Proto-Balto-Slavic *in, from the zero-grade *h₁n̥ of Proto-Indo-European *h₁én (“in”).

  Cognate with Lithuanian į, in (Eastern dialects), Old Prussian en.

• From Proto-Balto-Slavic [Term?], from o-grade *h₁ón of Proto-Indo-European *h₁én (“in”).

  Cognate with Latvian iekša (ie-kša), Ancient Greek ἐν (en) / ἐνί (ení, “in, within”), Proto-Italic *en (“in”), Proto-Celtic *eni / *en (“in”), Proto-Germanic *in (“in”).

Often assumed to come from the zero-grade. However there are no secure examples of *n̥ > Balto-Slavic *un (other examples like *sъto (often considered to be an Iranian borrowing) are all riddled with difficulties and unreliable), and the o-grade *h₁ón can be posited instead. See also *mъnogъ, *vъnukъ, *vъnъ, *vъtorъ and *kъ(n), *gъnati, *gъrnъ, *gъrdlo, *kъrma, *dǫti/*dъmǫ.

*vъ(n)

1. (+ locative) in, inside, within (stationary)
2. (+ accusative) in, into, inside (motion to)
3. (+ accusative) at (a moment in time)
4. (+ locative) in, during (a period of time)

Because of the law of open syllables, the final -n was normally dropped. But when combined with a stem that (originally?) began with a vowel, it was attached to the following word. (Example: *vъn *jejь > Bulgarian в нея "in her").

• *jьz

• East Slavic:
  • Old East Slavic: въ (vŭ), у (u), и (i)
    • Old Ruthenian: въ (v)
      • Belarusian: у (u), ў (ŭ)
      • Carpathian Rusyn: в (v), у (u)
      • Ukrainian: у (u), в (v)
    • Russian: в (v)
• South Slavic:
  • Old Church Slavonic: въ (vŭ)
  • Bulgarian: в (v)
  • Macedonian: во (vo)
  • Serbo-Croatian:

    Cyrillic script: у

    Latin script: u

    • Chakavian: u, va
    • Kajkavian: v, vu
  • Slovene: v
• West Slavic:
  • Old Czech: v
    • Czech: v
  • Old Polish: w, we
    • Masurian: w
    • Polish: w, we
    • Silesian: w, we, wy
  • Slovak: v
  • Pomeranian:
    • Kashubian: w, we, wew
    • Slovincian: w, we
  • Polabian: vå
  • Sorbian:
    • Upper Sorbian: w
    • Lower Sorbian: w

• Derksen, Rick (2008) “*vъ(n)”, in Etymological Dictionary of the Slavic Inherited Lexicon (Leiden Indo-European Etymological Dictionary Series; 4), Leiden, Boston: Brill, →ISBN, →ISSN, page 530
• Anikin, A. E. (2011) “в, во”, in Русский этимологический словарь [Russian Etymological Dictionary] (in Russian), numbers 5 (буба – вакштаф), Moscow: Znak, →ISBN, page 304
• Vasmer, Max (1964–1973) “в”, in Oleg Trubachyov, transl., Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress