The s or k , when initial in a conjunct consonant, are elided before a jhal (all consonant except semi-vowels and nasals) affix and at the end of a word.,
A conjunct consonant, having स for क as its first member, when coming at the end of a Pada, or when followed by a jhal beginning affix, loses its स् or क् ॥ Thus from the root लस्ज् we have लग्नः and लग्नवान् before the jhal affix त and तवत्; the substitution of न् for त् is considered asiddha for this purpose 8.2.1. So also साधुलक् at the end of a Pada. Similarly मग्नः मग्नवान्, साधुमक् from मस्ज् ॥ So also with initial क्, as तट् from तक्ष्; so also तष्टः, तष्टवान्, काष्ठतट् ॥
Vart:- It should be rather stated that \before a jhal affix included in the pratyahara सङ्\ ॥ The सङ् is a pratyahara formed with the स of सन् 3.1.5 and the ङ् of महिङ् 3.4.78. It thus includes all the krit affixes, and dhatu affixes i. e. affixes which come after a verb and not the Taddhita or the Feminine affixes. This Vartika applies to all the preceding sutras of this sub-division and is of use in the following places.
गिरोऽभोधिर्द्विष्टरां च दृषत्स्थः काष्ठशक्स्थिरः । कुञ्चाघुर्येति मा स्मैषु सत्वादीनि भवांत्विति ॥
So that in गिरः there should not be the optional ल by 8.2.21. In अभोधिः, the स् of अभस् is not elided before धि as required by 8.2.25. In द्विष्टरां the rule 8.2.27 does not apply. In दृषत्स्थः the rule 8.2.26 does not apply. In काष्ठशक्स्थिरः the rule 8.2.29 is non-applicable. In क्रुञ्चा the rule 8.2.30 does not apply. In धुर्यः (धुरं वहति) there is not lengthening by 8.2.77. Thus काष्ठशक्स्थाता ॥ Here क् would require to be elided as initial in a consonant, followed by a jhal consonant थ ॥ काष्ठशक् is formed by adding क्विप् to शक् ॥ But according to Patanjali, there can be formed no valid word from शक् with क्विप्, a fortiori, no such word can be formed as काष्ठशक्स्थाता (काष्ठशकि तिष्ठति).
In वास्यर्थम्, काक्यर्थम्, the स् and क् are not elided, because य् is a Bahiranga substitute and asiddha, and the word वास्य् काक्य् are not considered as Pada, ending in a conjunct consonant.
Why do we say \of स् and क्\? Observe नर्नर्त्ति, वर्वर्त्ति ॥
Why do we say 'initial in a conjunct consonant'? Observe पयः शक् ॥
Why do we say 'at the end of a word'? Observe तक्षिता, तक्षकः ॥,