Verbs in Proto-Slavic closely resembled those of attested Old Church Slavonic. They descended from the Proto-Indo-European verbal system, but had been simplified and altered substantially.
There were three tenses: present, imperfect and aorist. There were scarcely any remnants of the perfect, but the verb *věděti (“to know”) preserved a few traces. All moods except for the indicative and imperative (from the PIE optative) had disappeared. No voice was distinguished; all verbs were grammatically active. However, there were five participles: present active and passive, past active and passive, and resultative.
Slavic verbs developed a new system of aspect independent of the Indo-European aspectual system. In this system, verbs are inherently perfective (completed) or imperfective (incomplete, ongoing). Various pairs of perfective and imperfective verbs existed, with widely varying means of derivation. Some used prefixes to derive one from the other, some used different conjugational suffixes. Some pairs were suppletive and had equivalent verbs with different roots, and some had more than one counterpart. A number of verbs stood "alone" and had no counterpart at all. All in all, this system resembled that of early Proto-Indo-European, in which many roots had only one or two aspect stems and a few had more than one for the same aspect.
Proto-Slavic verbs generally have two stems, which are the base from which all other forms are created: the present stem and the infinitive stem. The present stem formed the basis of the present, imperfect and imperative, while the infinitive stem formed the infinitive, aorist and participles.
In Old Church Slavonic grammars, verbs are traditionally divided into four (or five) conjugation classes, depending on the present stem. However, this division ignores the formation of the infinitive stem. The following table shows the main classes of verbs in Proto-Slavic, along with their traditional OCS conjugation classes.
Five verbs remained in Proto-Slavic which retained the athematic endings inherited from Proto-Indo-European. The present-tense endings lack their initial vowel and are attached directly to the root/stem, often with consonant changes. Also characteristic is the first-person singular present ending -mь instead of -ǫ. The following verbs are reconstructable:
- *byti (“to be”) was suppletive and had a different root *es- in the present. It also had a unique set of forms to express future tense, based on a stem *bǫd-.
- *dati (“to give”) had reduplication in the present tense, *dad-. This was not apparent in most forms, however, as the -d- usually dropped before the endings.
- *ěsti (“to eat”)
- *jьměti (“to have”) had the present stem *jьma-.
- *věděti (“to know”) descended from a PIE perfect, and retained a unique ending -ě in the first-person singular present, next to -mь.
Consonant root stems
These verbs had no explicit stem-forming suffix; the stem was equal to the root and ended in a consonant. They were the descendants of PIE "thematic" verbs. This class was unproductive and contained only old inherited verbs. Several verbs also showed ablaut in the root or had other irregularities.
There were two subclasses:
- Verbs with an infinitive in *-ti. The infinitive ending was attached directly to the root, causing the root-final consonant to either disappear or change in many cases.
- Verbs with an infinitive in *-ati.
Vowel root stems
These verbs were parallel to the consonant root stems. The root either ended in -j- itself, or ended in a vowel or liquid diphthong to which -j- was added. This class was likewise unproductive and a few verbs still displayed ablaut.
There were two subclasses, parallel to the consonant root stems:
- Verbs with an infinitive in *-ti. The -j- was dropped in the aorist/infinitive stem, and the preceding vowel often underwent changes.
- Verbs with an infinitive in *-jati.
- *děti (“to put”) had a present stem with reduplication: *deď- (< *ded-j-).
Verbs with suffix -aj- and -ěj-
These verbs have -aj- or -ěj- suffixed to the root. The -j- disappears in the infinitive/aorist stem, thus resulting in *-ati and *-ěti.
- Verbs in -ěj- were mostly stative verbs. Remained somewhat productive.
- Verbs in -aj- were originally denominative (from nouns) or factitive (from adjectives). The class remained very productive and regularly formed new imperfective verbs.
Verbs with suffix -j/a- and -uj/ova-
These two types have -j- and -uj- in the present stem, which alternate with the infinitive in *-ati and *-ovati. The former type remained somewhat productive but the latter type was especially productive and still remains so in the modern Slavic languages.
Verbs with suffix -n-
These verbs had presents in -n-, infinitives in *-nǫti. They descended from various verbs with -n- suffixes in PIE, including presumably the n-infix verbs. This class remained productive in Proto-Slavic to form new perfective verbs. There were some older imperfectives in this class as well.
There were two subclasses:
- Verbs that retained -nǫ- in the aorist and participles.
- Verbs that had no -nǫ- in the aorist or participles, retaining it only in the infinitive. The verb *stati (“to stand”) belonged here too, but had no suffix in the infinitive either.
Verbs with suffix -i-
These verbs had -i- in all forms, and had special endings in the present tense. This class was widely productive and usually remains so in modern Slavic languages. It contained a variety of Indo-European formations which merged, including causatives/iteratives and denominatives.
Verbs with suffix -i/ě- and -i/a-
These verbs had -i- in the present tense like the class above, but -ě- in the infinitive. It was a small class containing mostly stative verbs. There were some verbs in this group that have -a- preceded by a palatal consonant rather than -ě-. This was a result of the change ě > a when preceded by a palatal consonant, which happened during the Slavic period.
Certain endings such as the infinitive *-ti often caused certain stem-final consonants to drop or change. Here is a list:
|All verbs in -j- (incl. -aj-, -ěj-)
*pěti (“to sing”)
|-j- is lost before all consonants.|
|-v- is lost before all consonants.|
|-ьm- and -ьn- become -ę- before all consonants.|
|-p- and -b- are lost before -t-.|
|-z- becomes -s- before -t-.|
|-t- and -d- become -s- before -t-, may undergo iotation to -ť- and -ď-.|
|-k- and -g- become -ť- before -t-.
May undergo 1st palatalization/iotation to -č- and -ž- or 2nd palatalization to -c- and -dz-.