In modern Sanskrit the affix kvasu optionally replaces liṭ (Perfect Tense) after the verbs sad , vas and śru with a past signification.,
This being a substitute only, the original affix लिट् also comes after these verbs. As, उपसेदिवान् कौत्सः पाणिनिम् 'Kautsa served Panini'. In the alternative, the proper affixes of the past tense will be employed. As उपासदत् 'he served'; उपासीदत् and उपससाद्.
So also अनूषिवान् कौत्सः पाणिनिम्. In the alternative we have अन्ववासीत्, अन्ववसत् and अनूवास.
So also उपशुश्रुवान् कौत्सः पाणिनिम्. In the alternative we have उपाश्रौषीत्, उपाश्रुणोत् and उपशुश्राव.
The affix क्वसु is employed with the force of लुङ्, and लङ् also.,