After a sarvanāmasthāna -- sarvādīni sarvanāmāni [[1.1.27]], when it signifies the cause of an action and the word hetu is used with it, the 3rd case-affix is used, as well as the 6th.,
Ex. कस्य or केन हेतुना वसति 'for the sake of what does he live?' यस्य or येन हेतुना वसति ।
Vart:- When the words निमित्त or कारण are so used, almost all the case affixes may be employed; as किं निमित्तं or केन निमित्तेन or कस्मै निमित्ताय or कस्मान्निमित्तात् or कस्मिन् निमित्ते वसति. Similarly with the words karana and hetu. Here also the word हेतु does not mean the word-form hetu 1.1.68 bub its synonyms also; as किं प्रयोजनं or केन प्रयोजनेन or कस्मै प्रयोजनाय &c., वसति.,