sí adv. "now" (Nam, RGEO:67, LR:47, SD:310, VT43:34, VT49:18, PE17:94), sin (SI, LR:47) or sín _(SD:247, 310) _before vowels. Compare the distribution of a/an in English, though in his Quenya version of Hail Mary, Tolkien used sí also before a vowel (sí ar "now and", VT43:28). Si, a short (or incompletely annotated) form of sí (VT43:26, 34). In Fíriel's Song, sí is translated "here".
sin (2) adv., a form of sí "now" (q.v.) often occurring before vowels; also sín (SI). However, sí itself (q.v.) may also appear before a vowel.