सोपानम् १८: पद-कार्यम्, त्रैपादी & चतुर्लकार-सिद्धिः (Word Finalization & 36 Living Forms)
Sopāna 18: Applying सुप्तिङन्तं पदम् (१.४.१४), अतो गुणे (६.१.९७), and अतो दीर्घो यञि (७.३.१०१) to forge all 36 living verbal forms across the 4 Sārvadhātuka Lakāras (लट्, लोट्, लङ्, विधिलिङ्) under the sovereign rule of पूर्वत्रासिद्धम् (८.२.१)
१. दर्शनम् — पद-व्यवस्था, त्रैपादी & ३६-रूप-सिद्धिः (The Architectural Finale)
"सुप्तिङन्तं पदम्। अपदं न प्रयुञ्जीत॥"
(Aṣṭādhyāyī 1.4.14 & Mahābhāṣya Maxim)
No bare root or affix is permitted in Sanskrit discourse. Every linguistic unit must be crowned with case or personal terminations to become an unshakeable, executable Pada.
१.४.१४ सुप्तिङन्तं पदम्
A stem coupled with Tiṅ endings (or Sup nominal cases) acquires the sovereign designation of a Pada. Only Padas enter syntactic composition.
अतो गुणे vs अतो दीर्घो यञि
६.१.९७ अतो गुणे merges stem 'a' + 'anti' into single 'a' (भवन्ति), while ७.३.१०१ अतो दीर्घो यञि lengthens 'a' to 'ā' before Yañ (भवामि, भवावः, भवामः).
८.२.१ पूर्वत्रासिद्धम्
The final 3 quarters of Chapter 8 execute as a one-way mirror. Rules here (such as Visarga and Ṇatva) are invisible to earlier rules, guaranteeing execution termination.