-о

• -е (-e) (after palatalized consonants, hushing consonants and ц)

• -о: IPA^(key): [ə]
• -о́: IPA^(key): [ˈo]

-о or -о́ • (-o or -ó)

1. Suffix added to adjectives to produce adverbs; highly productive

   ‎однозна́чный (odnoznáčnyj, “unambiguous”) + ‎-о (-o) → ‎однозна́чно (odnoznáčno, “unambiguously”)

   ‎бы́стрый (býstryj, “quick, fast”) + ‎-о (-o) → ‎бы́стро (býstro, “quickly”)

   ‎сме́лый (smélyj, “brave”) + ‎-о (-o) → ‎сме́ло (smélo, “bravely”)

   ‎го́рький (górʹkij, “bitter”) + ‎-о (-o) → ‎го́рько (górʹko, “bitterly”)

   ‎со́бственный (sóbstvennyj, “(one's) own”) + ‎-о (-o) → ‎со́бственно (sóbstvenno, “in fact, as a matter of fact”)

   ‎больно́й (bolʹnój, “sick, painful”) + ‎-о (-o) → ‎бо́льно (bólʹno, “painfully; extremely”)

   ‎глухо́й (gluxój, “deaf; dull, muffled”) + ‎-о (-o) → ‎глу́хо (glúxo, “deafly; dully, in a muffled manner”)

   ‎дорого́й (dorogój, “expensive, dear”) + ‎-о (-o) → ‎до́рого (dórogo, “expensively, dearly”)

   ‎ра́вный (rávnyj, “equal”) + ‎-о́ (-ó) → ‎равно́ (ravnó, “equally”)

   ‎смешно́й (smešnój, “funny, comic”) + ‎-о́ (-ó) → ‎смешно́ (smešnó, “in a funny manner, comically”)

   ‎у́мный (úmnyj, “clever, wise”) + ‎-о́ (-ó) → ‎умно́ (umnó, “cleverly, wisely”)

   ‎лёгкий (ljóxkij, “light; easy”) + ‎-о́ (-ó) → ‎легко́ (lexkó, “lightly; easily”)

   ‎хоро́ший (xoróšij, “good”) + ‎-о́ (-ó) → ‎хорошо́ (xorošó, “well”)

   ‎горя́чий (gorjáčij, “hot; hot-tempered”) + ‎-о́ (-ó) → ‎горячо́ (gorjačó, “hotly, warmly; passionately”)

   ‎да́вний (dávnij, “old, ancient, bygone”) + ‎-о́ (-ó) → ‎давно́ (davnó, “long ago”)

• The ending is most commonly unstressed even when the adjective is ending-stressed, but is sometimes stressed. Most of the time, whether the adverbial ending is unstressed or stressed follows the corresponding short neuter singular form of the adjective.
• This ending replaces -е (-e) when stressed.

• -и (-i) (used to form adverbs from adjectives whose stem ends in -ск- or -ий-/-j-)