Proto-Indo-European root

The roots of the reconstructed Proto-Indo-European language (PIE) are basic parts of words that carry a lexical meaning, so-called morphemes. PIE roots usually have verbal meaning like "eat" or "run". Roots never occur alone in the language. Complete inflected words like verbs, nouns or adjectives are formed by adding further morphemes to a root.

Word formation

Typically, a root plus a suffix forms a stem, and adding an ending forms a word.

For example, *bʰéreti 'he carries' can be split into the root *bʰer- 'to carry', the suffix *-e- 'imperfective aspect' and the ending *-ti 'present tense, third person singular'.

The suffix is sometimes missing, which has been interpreted as a zero suffix. Words with zero suffix are termed root verbs and root nouns. Beyond this basic structure, there is the nasal infix, a present tense marker, and reduplication, a sort of prefix with a number of grammatical and derivational functions.

Finite verbs

Verbal suffixes, including the zero suffix, convey grammatical information about tense and aspect, two grammatical categories that are not clearly distinguished. Present and aorist are universally recognised, while some of the other aspects remain controversial. Two of the four moods, the subjunctive and the optative, are also formed with suffixes, which sometimes results in forms with two consecutive suffixes: *bʰér-e-e-ti > *bʰérēti 'he would carry', with the first *e being the present tense marker, and the second the subjunctive marker. Reduplication can mark the present and the perfect.