Far more work has gone into reconstructing PIE than any other proto-language, and it is by far the best understood of all proto-languages of its age. The vast majority of linguistic work during the 19th century was devoted to the reconstruction of PIE or its daughter proto-languages (such as Proto-Germanic and Proto-Indo-Iranian), and most of the modern techniques of linguistic reconstruction (such as the comparative method) were developed as a result. These methods supply all current knowledge concerning PIE, since there is no written record of the language.
PIE is estimated to have been spoken as a single language from 4500 BC to 2500 BC during the Late Neolithic to Early Bronze Age, though estimates vary by more than a thousand years. According to the prevailing Kurgan hypothesis, the original homeland of the Proto-Indo-Europeans may have been in the Pontic–Caspian steppe of Eastern Europe. The linguistic reconstruction of PIE has also provided insight into the culture and religion of its speakers.
As speakers of Proto-Indo-European became isolated from each other through the Indo-European migrations, the regional dialects of Proto-Indo-European spoken by the various groups diverged from each other, as each dialect underwent different shifts in pronunciation (the Indo-European sound laws), morphology, and vocabulary. Thus these dialects slowly but eventually transformed into the known ancient Indo-European languages. From there, further linguistic divergence led to the evolution of their current descendants, the modern Indo-European languages. Today, the descendant languages, or daughter languages, of PIE with the most native speakers are Spanish, English, Portuguese, Hindustani (Hindi and Urdu), Bengali, Russian, Punjabi, German, Persian, French, Italian and Marathi.
PIE is believed to have had an elaborate system of morphology that included inflectional suffixes (analogous to English life, lives, life's, lives') as well as ablaut (vowel alterations, for example, as preserved in English sing, sang, sung) and accent. PIE nominals and pronouns had a complex system of declension, and verbs similarly had a complex system of conjugation. The PIE phonology, particles, numerals, and copula are also well-reconstructed.
An asterisk is used to mark reconstructed words, such as *wódr̥ 'water', *ḱwṓ 'dog', or *tréyes 'three (masculine)'; these forms are the reconstructed ancestors of the modern English words water, hound, and three.
The comparative method follows the Neogrammarian rule: the Indo-European sound laws apply without exception. The method compares languages and uses the sound laws to find a common ancestor. For example, compare the pairs of words in Italian and English: piede and foot, padre and father, pesce and fish. Since there is a consistent correspondence of the initial consonants that emerges far too frequently to be coincidental, one can assume that these languages stem from a common parent language.
Many consider William Jones, an Anglo-Welsh philologist and puisne judge in Bengal, to have begun Indo-European studies in 1786, when he postulated the common ancestry of Sanskrit, Latin, and Greek. However, he was not the first to make this observation. In the 1500s, European visitors to the Indian subcontinent became aware of similarities between Indo-Iranian languages and European languages, and as early as 1653 Marcus Zuerius van Boxhorn had published a proposal for a proto-language ("Scythian") for the following language families: Germanic, Romance, Greek, Baltic, Slavic, Celtic, and Iranian. In a memoir sent to the Académie des Inscriptions et Belles-Lettres in 1767 Gaston-Laurent Coeurdoux, a French Jesuit who spent all his life in India, had specifically demonstrated the analogy between Sanskrit and European languages. In the perspective of current academic consensus, Jones' work was less accurate than his predecessors', as he erroneously included Egyptian, Japanese and Chinese in the Indo-European languages, while omitting Hindi.
In 1818 Rasmus Christian Rask elaborated the set of correspondences to include other Indo-European languages, such as Sanskrit and Greek, and the full range of consonants involved. In 1816 Franz Bopp published On the System of Conjugation in Sanskrit in which he investigated a common origin of Sanskrit, Persian, Greek, Latin, and German. In 1833 he began publishing the .Comparative Grammar of Sanskrit, Zend, Greek, Latin, Lithuanian, Old Slavic, Gothic, and German
In 1822 Jacob Grimm formulated what became known as Grimm's law as a general rule in his Deutsche Grammatik. Grimm showed correlations between the Germanic and other Indo-European languages and demonstrated that sound change systematically transforms all words of a language. From the 1870s the Neogrammarians proposed that sound laws have no exceptions, as shown in Verner's law, published in 1876, which resolved apparent exceptions to Grimm's law by exploring the role that accent (stress) had played in language change.
By the early 1900s Indo-Europeanists had developed well-defined descriptions of PIE which scholars still accept today. Later, the discovery of the Anatolian and Tocharian languages added to the corpus of descendant languages. A new principle won wide acceptance in the laryngeal theory, which explained irregularities in the linguistic reconstruction of Proto-Indo-European phonology as the effects of hypothetical sounds which had disappeared from all documented languages, but which were later observed in excavated cuneiform tablets in Anatolian.
Julius Pokorny's Indogermanisches etymologisches Wörterbuch ("Indo-European Etymological Dictionary", 1959) gave a detailed, though conservative, overview of the lexical knowledge then accumulated. Kuryłowicz's 1956 Apophonie gave a better understanding of Indo-European ablaut. From the 1960s, knowledge of Anatolian became robust enough to establish its relationship to PIE.
Scholars have proposed multiple hypotheses about when, where, and by whom PIE was spoken. The Kurgan hypothesis, first put forward in 1956 by Marija Gimbutas, has become the most popular of these. It proposes that the Yamna culture associated with the kurgans (burial mounds) on the Pontic–Caspian steppe north of the Black Sea were the original speakers of PIE.
According to the theory, PIE became widespread because its speakers from the Kurgan culture could migrate into a vast area of Europe and Asia thanks to technologies such as the domestication of the horse, herding, and the use of wheeled vehicles.
The Lusitanian language is a marginally attested language found in the area of modern Portugal. The Venetic and Liburnian languages known from the North Adriatic region are sometimes classified as Italic.
The Paleo-Balkan languages, which occur in or near the Balkan peninsula, do not appear to be members of any of the subfamilies of PIE but are so poorly attested that proper classification of them is not possible. Albanian and Greek are the only surviving Indo-European languages in the group.
Proto-Indo-European phonology has been reconstructed in some detail. Notable features of the most widely accepted (but not uncontroversial) reconstruction include:
The Proto-Indo-European accent is reconstructed today as having had variable lexical stress, which could appear on any syllable and whose position often varied among different members of a paradigm (e.g. between singular and plural of a verbal paradigm). Stressed syllables received a higher pitch; therefore it is often said that PIE had a pitch accent. The location of the stress is associated with ablaut variations, especially between normal-grade vowels (/e/ and /o/) and zero-grade (i.e. lack of a vowel), but not entirely predictable from it.
The accent is best preserved in Vedic Sanskrit and (in the case of nouns) Ancient Greek, and indirectly attested in a number of phenomena in other IE languages. To account for mismatches between the accent of Vedic Sanskrit and Ancient Greek, as well as a few other phenomena, a few historical linguists prefer to reconstruct PIE as a tone language where each morpheme had an inherent tone; the sequence of tones in a word then evolved, according to that hypothesis, into the placement of lexical stress in different ways in different IE branches.
Proto-Indo-European roots were affix-lacking morphemes which carried the core lexical meaning of a word and were used to derive related words (e.g., "-friend-" in the English words "befriend", "friends", and "friend" by itself). Proto-Indo-European was a fusional language, in which inflectional morphemes signalled the grammatical relationships between words. This dependence on inflectional morphemes means that roots in PIE, unlike those found in English, were rarely found by themselves. A root plus a suffix formed a word stem, and a word stem plus a desinence (usually an ending) formed a word.
Many morphemes in Proto-Indo-European had short e as their inherent vowel; the Indo-European ablaut is the change of this short e to short o, long e (ē), long o (ō), or no vowel. This variation in vowels occurred both within inflectional morphology (e.g., different grammatical forms of a noun or verb may have different vowels) and derivational morphology (e.g., a verb and an associated abstract verbal noun may have different vowels).
Categories that PIE distinguished through ablaut were often also identifiable by contrasting endings, but the loss of these endings in some later Indo-European languages has led them to use ablaut alone to identify grammatical categories, as in the Modern English words sing, sang, sung.
Proto-Indo-European pronouns are difficult to reconstruct, owing to their variety in later languages. PIE had personal pronouns in the first and second grammatical person, but not the third person, where demonstrative pronouns were used instead. The personal pronouns had their own unique forms and endings, and some had two distinct stems; this is most obvious in the first person singular where the two stems are still preserved in English I and me. There were also two varieties for the accusative, genitive and dative cases, a stressed and an enclitic form.
The following table shows a possible reconstruction of the PIE verb endings from Sihler, which largely represents the current consensus among Indo-Europeanists.
Rather than specifically 100, *ḱm̥tóm may originally have meant "a large number".
Proto-Indo-European particles could be used both as adverbs and postpositions, like *upo "under, below". The postpositions became prepositions in most daughter languages. Other reconstructible particles include negators (*ne, *mē), conjunctions (*kʷe "and", *wē "or" and others) and an interjection (*wai!, an expression of woe or agony).
Proto-Indo-European employed various means of deriving words from other words, or directly from verb roots.
Internal derivation was a process that derived new words through changes in accent and ablaut alone. It was not as productive as external (affixing) derivation, but is firmly established by the evidence of various later languages.
Possessive or associated adjectives could be created from nouns through internal derivation. Such words could be used directly as adjectives, or they could be turned back into a noun without any change in morphology, indicating someone or something characterised by the adjective. They could also be used as the second element of a compound. If the first element was a noun, this created an adjective that resembled a present participle in meaning, e.g. "having much rice" or "cutting trees". When turned back into nouns, such compounds were Bahuvrihis or semantically resembled agent nouns.
In thematic stems, creating a possessive adjective involved shifting the accent one syllable to the right, for example:
In athematic stems, there was a change in the accent/ablaut class. The known four classes followed an ordering, in which a derivation would shift the class one to the right:
The reason for this particular ordering of the classes in derivation is not known. Some examples:
A vrddhi derivation, named after the Sanskrit grammatical term, signified "of, belonging to, descended from". It was characterised by "upgrading" the root grade, from zero to full (e) or from full to lengthened (ē). When upgrading from zero to full grade, the vowel could sometimes be inserted in the "wrong" place, creating a different stem from the original full grade.
Adjectives with accent on the thematic vowel could be turned into nouns by moving the accent back onto the root. A zero grade root could remain so, or be "upgraded" to full grade like in a vrddhi derivative. Some examples:
This kind of derivation is likely related to the possessive adjectives, and can be seen as essentially the reverse of it.
The syntax of the older Indo-European languages has been studied in earnest since at least the late nineteenth century, by such scholars as Hermann Hirt and Berthold Delbrück. In the second half of the twentieth century, interest in the topic increased and led to reconstructions of Proto-Indo-European syntax.
Since all the early attested IE languages were inflectional, PIE is thought to have relied primarily on morphological markers, rather than word order, to signal syntactic relationships within sentences. Still, a default (unmarked) word order is thought to have existed in PIE. This was reconstructed by Jacob Wackernagel as being subject–verb–object (SVO), based on evidence in Vedic Sanskrit, and the SVO hypothesis still has some adherents, but as of 2015 the "broad consensus" among PIE scholars is that PIE would have been a subject–object–verb (SOV) language.
The SOV default word order with other orders used to express emphasis (e.g., verb–subject–object to emphasise the verb) is attested in Old Indic, Old Iranian, Old Latin and Hittite, while traces of it can be found in the enclitic personal pronouns of the Tocharian languages. A shift from OV to VO order is posited to have occurred in late PIE since many of the descendant languages have this order: modern Greek, Romance and Albanian prefer SVO, Insular Celtic has VSO as the default order, and even the Anatolian languages show some signs of this word order shift. The inconsistent order preference in Baltic, Slavic and Germanic can be attributed to contact with outside OV languages.
The Ridley Scott film Prometheus features an android named "David" (played by Michael Fassbender) who learns Proto-Indo-European to communicate with the "Engineer", an extraterrestrial whose race may have created humans. David practices PIE by reciting Schleicher's fable and goes on to attempt communication with the Engineer through PIE. Linguist Dr Anil Biltoo created the film's reconstructed dialogue and had an onscreen role teaching David Schleicher's fable.
The 2016 video game Far Cry Primal presented dialects of a reconstructed Proto-Indo-European language. Linguists constructed three dialects—Wenja, Udam and Izila—one for each of the three featured tribes.