arose not in Latin but in Indo-European in words which had originally contained the Normal form of root but in which the Accent in that period fell on the following syllable, and had thus reduced the root-syllable to a Weak form.
We find similar variation due ultimately to the same cause in suffixes (§ 226–243), as in -ter- -tr-, -en- and -n-, -meno- and -mno-.
§ 70. The commonest case of this weakening of syllables through their coming to stand in an unaccented position is the Participle in -tós, where, as we have seen in § 60 (2) (b), the suffix always bore the accent. Hence e.g. the partc. of I.Eu. *ten-iō ‘I stretch’ (Gr. τείνω, Lat. tendo) became not *ten-tós but *tntós which appears in Gr. τατός, Sans. tatás, for in Greek and Sanskrit n and m both became a. In Latin however n always became en, so the Latin form is tentus. The Verbal Nouns ending with the suffix -tí-, which was always accented, also show the Weak form of the root, as Gr. φάτις, from the root of Lat. fama, Gr. φᾱμί, φημί, or Gr. πίστις beside πείθω, or Lat. ratis beside rē-ri ‘to think’ (§ 216).
§ 71. From roots whose Normal form contained ā, ē, or ō, the Weak form in Indo-European had the neutral vowel ə which became short a in Latin (§ 55). This explains such groups of forms as the following:
Root dhē- (sometimes in a longer form dhēk̑-) ‘to make, put,’ Gr. τί‐θη‐μι, ἔ‐θηκ‐α.
: Sans. da-dhā-mi.
: Lat. fēcī beside facio.
