Their grammatical forms and meanings have been reconstructed by modern linguists, based on similarities found across all Indo-European languages.
Two declensions ended in a vowel (*-o/-e[note 1]) and are called thematic; they were more regular and became more common during the history of PIE and its older daughter languages.
Variation in the position of the accent likewise occurred in both derivation and inflection, and is often considered part of the ablaut system (which is described in more detail below).
[3] Already by late PIE times, this system was extensively simplified, and daughter languages show a steady trend towards more and more regularization and simplification.
Far more simplification occurred in the late PIE nominal system than in the verbal system, where the original PIE ablaut variations were maintained essentially intact well into the recorded history of conservative daughter languages such as Sanskrit and Ancient Greek, as well as in the Germanic languages (in the form of strong verbs).
An example is *ni-sd-ó-s 'nest', derived from the verbal root *sed- 'sit' by adding a local prefix and thus meaning "where [the bird] sits down" or the like.
Stems ending in *i or *u such as *men-ti- are consonantic (i.e. athematic) because the *i is just the vocalic form of the glide *y, the full grade of the suffix being *-tey-.
[13] This view is supported by the existence of certain classes of Latin and Ancient Greek adjectives which inflect only for two sets of endings: one for masculine and feminine, the other for neuter.
[17] Remnants of this period exist in (for instance) the eh₂-stems, ih₂-stems, uh₂-stems and bare h₂-stems, which are found in daughter languages as ā-, ī-, ū- and a-stems, respectively.
Nonetheless, clear traces of the earlier system are seen especially in Sanskrit, where ī-stems and ū-stems still exist as distinct classes comprising largely feminine nouns.
Grammatical gender correlates only partially with sex, and almost exclusively when it relates to humans and domesticated animals.
[18] An alternative hypothesis to the two-gender view is that Proto-Anatolian inherited a three-gender PIE system, and subsequently Hittite and other Anatolian languages eliminated the feminine by merging it with the masculine.
Sihler (1995)[20] remains closest to the data, often reconstructing multiple forms when daughter languages show divergent outcomes.
Ringe (2006)[21] is somewhat more speculative, willing to assume analogical changes in some cases to explain divergent outcomes from a single source form.
The thematic vowel *-o- ablauts to *-e- only in word-final position in the vocative singular, and before *h₂ in the neuter nominative and accusative plural.
[22] This reconstruction does not give separate tables for the thematic and athematic endings, assuming that they were originally the same and only differentiated in daughter languages.
[24] Early PIE nouns had complex patterns of ablation according to which the root, the stem and the ending all showed ablaut variations.
Polysyllabic athematic nominals (type R+S+E) exhibit four characteristic patterns, which include accent and ablaut alternations throughout the paradigm between the root, the stem and the ending.
[29] Some[16] also list mesostatic (meso = middle) and teleutostatic types, with the accent fixed on the suffix and the ending, respectively, but their existence in PIE is disputed.
[30] The classes can then be grouped into three static (acrostatic, mesostatic, teleutostatic) and three or four mobile (proterokinetic, hysterokinetic, amphikinetic, holokinetic) paradigms.
By late PIE, the above system had been already significantly eroded, with one of the root ablaut grades tending to be extended throughout the paradigm.
Similarly, PIE *ǵónu, *ǵnéus can be reconstructed for 'knee' from Ancient Greek gónu and Old English cnēo.
An ablauting paradigm *pōds, *ped- can also clearly be reconstructed from 'foot', based on Greek pous gen. podós (< *pō(d)s, *pod-) vs. Latin pēs gen. pedis (< *ped-) vs. Old English fōt (< *pōd-), with differing ablaut grades among cognate forms in different languages.
All except the Latin form suggest a masculine u-stem with non-ablauting PIE root *ǵen-, but certain irregularities (the position of the accent, the unexpected feminine ā-stem form in Latin, the unexpected Gothic stem kinn- < ǵenw-, the ablaut found in Greek gnáthos 'jaw' < PIE *ǵnHdʰ-, Lithuanian žándas 'jawbone' < *ǵonHdʰ-os) suggest an original ablauting neuter noun *ǵénu, *ǵnéus in early PIE.
Another example is *nokʷts 'night'; an acrostatic root paradigm might be expected based on the form, but the consistent stem *nokʷt- is found throughout the family.
Leftward shift of the accent could turn an agentive word into a resultative one, for example *tomós 'sharp', but *tómos 'a slice' (from *tem- 'to cut'); *bʰorós 'carrier', but *bʰóros 'burden' (from *bʰer- 'carry').
A special type of ablaut alternation was vṛddhi derivation, which typically lengthened a vowel, signifying "of, belonging to, descended from".
Endocentric or determinative compounds denote subclasses of their head (usually the second part), as in English "smalltalk" or "blackbird".
The cognates derived from these roots in different daughter languages often do not agree in formation, but show certain characteristic properties:[41][42][note 5] The comparative form ("bigger, more beautiful") could be formed by replacing an adjective's suffix with *-yos-; the resulting word is amphikinetic: *meǵ-no-[43] 'big' (Latin magnus) → *méǵ-yos- 'bigger' (Latin maior, maius), weak cases *meǵ-is-.
[44][45] PIE probably expressed the superlative ("biggest, most beautiful") by adding a genitive plural noun to the adjective.