Indo-European copula copula (with corresponding voiceless and voiced fricatives) existed side by side in Germanic, and the involvement of a separate root "" is unnecessary. The root "*(s)teh-" meant "to stand". From this root comes the present stem of the so-called "substantive verb" in Irish and Scottish Gaelic, "tá" and "tha" respectively. On the absence of the initial s- in Celtic, see Indo-European s-mobile. In Latin, "stō, stare" retained the meaning "to stand", until local forms of Vulgar Latin began to use it as a copula in certain circumstances. Today, this survives in that several Romance languages (Galician-Portuguese, Spanish, Catalan) use it
Proto-Indo-European verbs system was fully formed; however, there is no consensus concerning what the inherited system looked like, and which Anatolian differences are innovations vs. archaisms. In general, the traditional "Cowgill–Rix" system reconstructs the following categories for Proto-Indo-European (excluding Hittite and other Anatolian languages): Some of these categories (e.g. the dual) tended to drop out in time, while some languages innovated new categories (e.g. the future tense). The original interpretation of the tense and aspect categories has been a thorny issue; see below. PIE verb morphology resembled, in many respects, that of nominals. A verb was formed by adding a suffix (S)