DISCLAIMER: This article is preliminary research on the part of its author (Paul Strack) and does not necessarily reflect the opinions of the owner of this site. Since the source material is complex and its interpretation can be subjective, multiple conclusions are possible.
There only two currently published questions in Sindarin (and none in its conceptual precursors): linnathol? “will you sing?” from 1969 (PE22/167) and man agorech? from the early 1950s (VT50/5), untranslated but almost certainly meaning “what have we done?” using the late-1940s, early-1950s 1st pl. inclusive pronominal suffix -ch (VT50/21-22). These two questions give us a fair amount of information, though. The first demonstrates that a question can be indicated by intonation alone, without any special question words. This second indicates that a question word at the beginning of the phrase like man “what” can be used to indicate a question. This is consistent with Quenya question formation.
There are, unfortunately, almost no attested question words in Sindarin; the aforementioned man “what, (?) who” is the only one. Fiona Jallings suggested a more comprehensive set of interrogative pronouns in her book A Fan’s Guide to Neo-Sindarin based on the contraction of various prepositions with man (FGNS/220-221). I recommended adopting her approach, though in my own list of interrogative pronouns I suggest *mallú for “when” rather than her *neman (mostly because I don’t think ne means “during”).
When responding to questions, you have several options. We don’t know the Sindarin word for “yes”, but many Neo-Sindarin writers now assume it would be *naw, the equivalent of Q. ná “yes, it is so” (PE22/166). For “no” you can use û (PE17/144), or if you prefer la-negation you can use the neologism *law. In response to a request you can say either athon “I will” (PE22/167) or avon “I won’t” (PE17/143; WJ/371). When answering for a group: athof “we will” or avof “we won’t”.