indeclinable adversative & interrogative particle, sometimes (originally, cp. puna “again, further”) merely connecting & continuing the story

1. (adversative) but, on the contrary Ja.i.222; Ja.ii.159; Vv-a.79 (correl. with tāva). ca pana “but” Ja.i.152; atha ca pana “and yet” DN.i.139; Ja.i.279; na kho pana “certainly not” Ja.i.151; vā pana “or else” Vin.i.83; Dhp.42; Snp.376, Snp.829
2. (in questions) then, now Ja.ii.4 (kiṃ p.), Ja.ii.159 (kahaṃ p.) Vv-a.21 (kena p.); Pv-a.27 (katamaṃ p.)
3. (conclusive or copulative) and, and now, further, moreover DN.i.139 (siyā kho p. be it now that…); Snp.23, Snp.393 Snp.396, Snp.670; Ja.i.278; Pv-a.3.

doublet of Sk. puna(ḥ) wiṭh diff. meaning (see puna), cp. Geiger, P.Gr. § 34