The evolution of Old Tamil into Middle Tamil, which is generally taken to have been completed by the 8th century, was characterised by a number of phonological and grammatical changes. In phonological terms, the most important shifts were the virtual disappearance of the aytam (ஃ), an old phoneme, the coalescence of the alveolar and dental nasals, and the transformation of the alveolar plosive into a rhotic. In grammar, the most important change was the emergence of the present tense. The present tense evolved out of the verb kil (கில்), meaning "to be possible" or "to befall". In Old Tamil, this verb was used as an aspect marker to indicate that an action was micro-durative, non-sustained or non-lasting, usually in combination with a time marker such as ṉ (ன்). In Middle Tamil, this usage evolved into a present tense marker – kiṉṟa (கின்ற) – which combined the old aspect and time markers.
From the period of the Pallava dynasty onwards, a number of Sanskrit loan-words entered Tamil, particularly in relation to political, religious and philosophical concepts. Sanskrit also influenced Tamil grammar, in the increased use of cases and in declined nouns becoming adjuncts of verbs, and phonology. The forms of writing in Tamil have developed through years. The Tamil script also changed in the period of Middle Tamil. Tamil Brahmi and Vaṭṭeḻuttu, into which it evolved, were the main scripts used in Old Tamil inscriptions. From the 8th century onwards, however, the Pallavas began using a new script, derived from the Pallava Grantha script which was used to write Sanskrit, which eventually replaced Vaṭṭeḻuttu.
^Rajam, V. S. (1985). "The Duration of an Action-Real or Aspectual? The Evolution of the Present Tense in Tamil". Journal of the American Oriental Society105 (2): 277. doi:10.2307/601707. JSTOR601707.edit at pp. 284–285