PIE verbs and nominals (nouns and adjectives) consist of three parts: The thematic vowel, if present, occurs at the end of the suffix (which may include other vowels or consonants) and before the ending: Athematic forms, by contrast, have a suffix ending in a consonant, or no suffix at all (or arguably a null suffix):[2] For several reasons, athematic forms are thought to be older, and the thematic vowel was likely an innovation of late PIE: Athematic paradigms (inflection patterns) are more "irregular", exhibiting ablaut and mobile accent, while the thematic paradigms can be seen as a simplification or regularisation of verbal and nominal grammar.
In the Anatolian languages, which were the earliest to split from PIE, thematic verbs are rare or absent.
In verbs, the thematic vowel is *e word-finally or when the following ending begins with a coronal obstruent (*t, *d, *dʰ or *s) and *o otherwise.
[citation needed] Here is the present active indicative paradigm of *bʰer- 'carry':[5] For comparison, here is an example of an athematic verb, *dewk- 'to draw'.
[8] The thematic presents in Western PIE also do not have quantitative ablaut, which indicates their relatively recent origin.
This all has caused some linguists to speculate that perfect and thematic present endings go back to a single Early PIE prototype.
Here is an example paradigm for *h₂ŕ̥tḱos 'bear', a thematic animate noun, supplemented by the neuter *h₂érh₃trom 'plough' for the nominative/accusative:[10] Again, athematic nouns show ablaut and accent shifts, mainly between the "strong" cases (nominative and vocative in all numbers, and accusative singular/dual) and the "weak" cases (all others).
[11] This caused an asymmetry between the valencies of transitive and intransitive verbs, summarized in the table below:[12] This theory was further developed by Beekes and Kortlandt, who assumed that the nominative syntax of old Indo-European languages was formed later and that the case system of the PIE language was primarily based on the ergative syntax.
[13] Similar theories that assume the ergative past of the PIE syntax have been formulated by Gamkrelidze and Ivanov[14] and Schmalstieg.
Matasović argued that the thematic o-stem nouns were the result of the nominalisation of adjectives, which in turn arose through the reinterpretation as nominative forms of original (attributively used) genitives of athematic (mostly deverbal) root-nouns.
This theory, like the previous one, would explain why there is much evidence in favour of original syncretism of the nominative and genitive singular in the o-stems.
[16] According to Jean Haudry o-stems originated from pronouns with a determining function that were suffixed to a nominal base, playing the role of a postpositional article.
[17] There exists a number of typological parallels for such a development: Thematic and athematic forms were passed on to the daughter languages of Proto-Indo-European.
Sanskrit grammar ordains a vikaraṇapratyaya (modificatory affix) between a verbal root and the tense-ending.
The distinction between thematic and athematic stems is especially apparent in the Greek verb; they fall into two classes that are marked by quite different personal endings.
For example, in the Ancient Greek verb τέμνω (témnō) 'cut', tem- is the root, and temn- is the stem or theme for the present tense.