Proto-Indo-European verb

Proto-Indo-European verbs had a complex system, with verbs categorized according to their aspect: stative, imperfective, or perfective. The system used multiple grammatical moods and voices, with verbs being conjugated according to person, number and tense.

The system of adding affixes to the base form of a verb (its root) allowed modifications that could form nouns, verbs or adjectives. The verbal system is clearly represented in Ancient Greek and Vedic Sanskrit, which closely correspond, in nearly all aspects of their verbal systems, and are two of the most well-understood of the early daughter languages of Proto-Indo-European.

Besides the addition of affixes, another way that vowels in the word could be modified was the Indo-European ablaut, which is still visible in languages such as the Germanic languages. For example, the vowel in the English (a Germanic language) verb to sing varies according to the conjugation of the verb: sing, sang and sung.

The system described here is known as the "Cowgill–Rix" system and, strictly speaking, applies only to what Don Ringe terms "Western Indo-European" (Western IE), i.e. IE excluding and especially Anatolian. The system also describes Tocharian fairly well, but encounters significant difficulties when applied to Hittite and the other Anatolian languages. In particular, despite the fact that the Anatolian languages are the earliest-attested IE languages, much of the complexity of the Cowgill–Rix system is absent from them. In addition, contrary to the situation with other languages with relatively simple verbal systems, such as Germanic, there is little or no evidence of the "missing" forms having ever existed. Furthermore, many of the forms that do exist have a significantly different meaning from elsewhere. For example, the PIE perfect/stative conjugation shows up simply as a present-tense conjugation known as the ḫi-present, with no clear meaning; on the other hand, the PIE nu-present, which in other languages is a primary verb suffix with no clear meaning, is in Hittite a productive secondary verb suffix that forms causative verbs. (On the other hand, Germanic, among others, has a class of present-tense verbs derived from PIE perfect/stative verbs, and both Germanic and Balto-Slavic have a class of secondary n- verbs with a clear meaning, derived originally from nu- and/or neH- verbs, so it is possible that many of the Anatolian differences are innovations.) It is generally accepted that the Anatolian languages diverged from other IE languages at a point somewhat before the Cowgill–Rix system was fully formed; however, there is no consensus concerning what the inherited system looked like, and which Anatolian differences are innovations vs. archaisms.

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