Morphology and syntax
Verbal inflection
The Proto-Indo-European verb had three aspects: imperfective, perfective, and stative. Aspect refers to the nature of an action as described by the speaker—e.g., an event occurring once, an event recurring repeatedly, a continuing process, or a state. The difference between English simple and “progressive” verb forms is largely one of aspect—e.g., “John wrote a letter yesterday” (implying that he finished it) versus “John was writing a letter yesterday” (describing an ongoing process, with no implication as to whether it was finished or not).
The imperfective aspect, traditionally called “present,” was used for repeated actions and for ongoing processes or states—e.g., *stí-stH2-(e)- ‘stand up more than once, be in the process of standing up,’ *mn̥-yé- ‘ponder, think,’ *H1es- ‘be.’ The perfective aspect, traditionally called “aorist,” expressed a single, completed occurrence of an action or process—e.g., *steH2- ‘stand up, come to a stop,’ *men- ‘think of, bring to mind.’ The stative aspect, traditionally called “perfect,” described states of the subject—e.g., *ste-stóH2- ‘be in a standing position,’ *me-món- ‘have in mind.’
Verb roots were by themselves either perfective (like *steH2- ‘stand’ and *men- ‘think’) or imperfective (like *H1es- ‘be’). This basic aspect, however, could be reversed by morphological devices such as ablaut, suffixation, and reduplication. The stative aspect was normally marked by reduplication and the 0-grade of the root in the indicative singular; it had personal endings that were partly distinct from those of the other two aspects.
From one aspect of a given verb the shape and even the existence of the other two aspects could not be predicted; for example, *H1es- ‘be’ had only the imperfective aspect. Ways of forming imperfectives were especially numerous and often involved, in addition to their imperfective aspectual meaning, some other notion, such as performing the action habitually or repeatedly (iterative), or causing someone else to perform it (causative). One root could thus have several imperfective stems; so to the root *H1er- ‘move’ there were at least a causative form, *H1r̥-new- ‘set in motion,’ and an iterative form, *H1r̥-sḱḥ- ‘go repeatedly.’
The Proto-Indo-European verb was also inflected for mood, by which speakers could indicate whether they were making statements or inquiries about matters of fact; making predictions, surmises, or wishes about the future or about unreal but imagined situations; or giving commands. Compare English “If John is home now (he is eating lunch)” with the verb is in the indicative mood, discussing a matter of fact, with “If John were home now (he would be eating lunch)” with the verb were in the subjunctive mood, describing an unreal situation. There were two Proto-Indo-European suffixes expressing mood: -e- alternating with -o- for the subjunctive, corresponding roughly in meaning to the English auxiliaries ‘shall’ and ‘will,’ and -yeH1- alternating with -iH1- for the optative, corresponding roughly to English ‘should’ and ‘would.’ Verbs without one of these two suffixes were marked for mood and tense by their personal endings alone.

These personal endings basically expressed the person and number of the verb’s subject, as in Latin amō ‘I love,’ amās ‘you (singular) love,’ amat ‘he or she loves,’ amāmus ‘we love,’ and so on. In the imperfective and perfective aspects there were two sets of endings, distinguishing two voices: active, in which typically the subject was not affected by the action, and mediopassive, in which typically the subject was affected, directly or indirectly. Thus, Sanskrit active yájati and mediopassive yájate both mean ‘he sacrifices,’ but the former is said of a priest who performs a sacrifice for the benefit of another, while the latter is said of a layman who hires a priest to perform a sacrifice. In the stative aspect there was originally no distinction of voice.
To mark mood and tense, imperfective verbs that did not have a mood suffix distinguished three subtypes of active and mediopassive endings: imperative, primary, and secondary. Verbs with imperative endings belonged to the imperative mood (used for commands)—e.g., *H1s-dhí ‘be (singular),’ *H1és-tu ‘let him be.’ Verbs with primary endings were marked as non-past (present or future) in tense and indicative in mood—e.g., *H1és-ti ‘he is.’ (Indicative mood signifies objective statements and questions.) Verbs with secondary endings were unmarked for tense and mood but were normally used as past indicatives (e.g., *H1és-t ‘he was,’ *gwhén-t ‘he slew’) and to fill out gaps in the imperative paradigm (e.g., *H1és-te or *H1s-té ‘you [plural] were,’ but also ‘be [plural]’; *gwhén-te or *gwhn̥-té ‘you [plural] slew,’ but also ‘slay [plural]’). To mark such forms unambiguously as past indicatives, an augment, usually consisting of the vowel e, could be prefixed—e.g., *é-gwhen-t ‘he slew,’ *é-H1es-t ‘he was.’
Verbs in the perfective aspect without a mood suffix did not occur with primary endings and thus lacked a true present tense. Verbs in the stative aspect substituted a distinctive set of endings for those of the primary set but apparently used the imperative and secondary endings in the usual way to form a stative imperative and a stative past indicative.
Nominal inflection
The inflectional categories of the noun were case, number, and gender. Eight cases can be reconstructed: nominative, for the subject of a verb; accusative, for the direct object; genitive, for the relations expressed by English of; dative, corresponding to the English preposition to, as in “give a prize to the winner”; locative, corresponding to at, in; ablative, from; instrumental, with; and vocative, used for the person being addressed. For examples of some of these, see Click Here to see full-size tableTable 2. Besides singular and plural number, there was a dual number for referring to two items. Each noun belonged to one of three genders: masculine, to which belonged most nouns designating male creatures; feminine, to which belonged most names of female creatures; and neuter, to which belonged only a few words for individual adult living creatures. The gender of nouns not designating living creatures was only partly predictable from their meaning.
Adjectives were nounlike words that varied in gender according to the gender of another noun with which they were in agreement, or, if used by themselves, according to the sex of the entity to which they referred; thus, Latin bonus sermō ‘good speech’ (masculine), bona aetās ‘good age’ (feminine), bonum cor ‘good heart’ (neuter), or bonus ‘a good man,’ bona ‘a good woman,’ bonum ‘a good thing.’ The neuter of an adjective was often identical with the masculine except for having different endings in the nominative and accusative cases. Feminine gender was either completely identical with the masculine or derived from it by means of a suffix, the two commonest being *-eH2- and *-iH2- (*-yeH2-).
Demonstrative, interrogative, relative, and indefinite pronouns were inflected like adjectives, with some special endings. Personal pronouns were inflected very differently. They lacked the category of gender, and they marked number and case (in part) not by endings but by different stems, as is still seen in English singular nominative “I,” but oblique “my,” “me”; plural nominative “we,” but plural oblique “our,” “us.” (The oblique is any case other than nominative or vocative.)
Syntax
Some notable features of Proto-Indo-European syntax were the non-ergative case system, in which the subject of an intransitive verb received the same case marking as the subject (rather than the object) of a transitive verb; concord (agreement) in case, number, and gender between adjective and noun; and the use of singular verbs with neuter plural subjects, as in Greek pánta rheĩ ‘all things flow,’ with the same (singular) verb as ho pótamos rheĩ ‘the river (masculine) flows,’ contrasting with hoi pótamoi rhéousi ‘the rivers flow’ (indicating that neuter plurals were originally collectives and grammatically singular). Proto-Indo-European word order was flexible, but basic declarative sentences typically had the structure subject–object–verb (SOV).
Lexicon and culture
Much less is known about the parent language’s vocabulary than about its phonology and grammar. Sounds and grammatical categories do not easily disappear or undergo radical change in so many daughter languages that their former existence can no longer be detected. It is relatively easy, however, for an individual word to disappear or shift meaning in so many daughter languages that its existence or meaning in the parent language cannot be confidently inferred. Hence, from the linguistic evidence alone, scholars can never say that Proto-Indo-European lacked a word for any particular concept; they can only state the probability that certain items did exist and from these items make inferences about the culture and location in time and space of the speakers of Proto-Indo-European.
Thus is it supposed that the Proto-Indo-European community knew and talked about dogs (*ḱwón-), horses (*H1éḱwo-), sheep (*H3éwi-), and almost certainly cows (*gwów-) and pigs (*súH-). Probably all these animals were domesticated. At least one cereal grain was known (*yéwo-), and at least one metal (*H2éyos). There were vehicles (*wóǵho-) with wheels (*kwékwlo-), pulled by teams joined by yokes (*yugó-). Honey was known, and it probably formed the basis of an alcoholic drink (*mélit-, *médhu) related to the English mead. Numerals up through 100 (*ḱm̥tóm) were in use. All this suggests a people with a well-developed Neolithic (characterized by simple agriculture and polished stone tools) or even Chalcolithic (copper- or bronze-using) technology.
The divergence of Indo-European languages
Linguists have not found a reliable and precise way to determine from linguistic evidence alone the date at which any set of related languages must have begun diverging. Computational methods for calculating the “time depths” of language families have been proposed, but they have not been shown to yield reliable results. The best that can be done is to estimate the degree of difference between the languages in question, taking into account all that is known about them, and then compare this estimate with the estimated degrees of difference within families of languages—such as the Romance family—whose actual time of divergence is approximately known. Using this sort of “dead reckoning,” most linguists agree that the earliest attested Indo-European languages—Anatolian, Indo-Iranian, and Greek—are different enough that the parent language must have been split into several distinct languages before 3000 bce, but similar enough that the first split into separate languages is not likely to have been much earlier than about 4500 bce.
For further progress the linguistic findings must be correlated with archaeological evidence. Linguistic, historical, and geographic considerations suggest that the speakers of Proto-Indo-European were a relatively small and homogeneous Eurasian population group that underwent significant expansion and fragmentation in the period around 4000 bce. Many scholars identify the Indo-Europeans with the bearers of what has been called the “Kurgan (Barrow) culture” of the Black Sea and the Caucasus and west of the Urals. The Kurgan culture, however, was only one of a number of related steppe cultures extending across the entire Black Sea–Caspian Sea region, an area that was transformed after 4000 bce by the advent of horse-drawn wheeled vehicles and related innovations. It is probably best, therefore, to follow J.P. Mallory (In Search of the Indo-Europeans [1989]) in locating the speakers of Proto-Indo-European among the populations of this region but not to attempt a more precise identification until further evidence is available. A radically different theory, according to which the Indo-European spread began in Asia Minor about 7000 bce, is difficult to square with the linguistic facts.
A remote relationship of Indo-European to the Uralic languages is possible. Geographically, the earliest reconstructible locations of the two families are contiguous; there are strong resemblances in a number of basic grammatical elements, including personal, demonstrative, interrogative, and relative pronouns, personal endings of verbs, the accusative case ending -m, and a very few words, such as those for ‘water’ and ‘name’; typologically, the families are fairly similar—e.g., both have many suffixes, but few or no prefixes or infixes (elements inserted within words). On the whole, however, the lexical resemblances between Indo-European and Uralic are very sparse; the two families, if they are related at all, must have separated thousands of years before the breakup of Proto-Indo-European.
If Indo-European is related to other language families—e.g., to Afro-Asiatic (which includes the Semitic languages) or to Kartvelian (which includes Georgian)—it must have diverged from them much earlier than it diverged from Uralic, because the number of cogent resemblances is still smaller. There is no significant evidence at present for a “Nostratic” superfamily embracing these and other groups.