The Taittiriya Upanishad is a Sanskrit text. It is one of the older, "primary" Upanishads commented upon by Shankara. It is associated with the Taittiriya school of the Yajurveda.
Taittiriya Upanishad by Vidyasankar S
Ch 4: Mandukya Upanishad by Swamini Atmaprakashananda
Chapter 17 - Uddhava Gita (Tamil) by Swami Guruparananda
Ch 09: Bhagavad Gita (Tamil) by Swami Guruparananda
Gita Study Group - Swami Tyagananda