Eldamo glosses the Quenya word to as an adverb meaning "as a rule" based on the Q sentence lairesse nihare to tarassi, yo unta hrívesse landannar in PE22:125; the sentence is translated by Tolkien as "In summer I live in the hills as a rule, and come down to the plains in the winter."
See http://eldamo.org/content/words/word-771384677.html
I don't think to means "as a rule"; I read it simply to be the superessive preposition "on, on the surface of"; see the rough list of prepositions in LVS 14 in PE22:168:
tŏ on, [???] above, on [???}
I think "as a rule" is not verbatim in the sentence but expressed by the tense and meaning of the verb har- "dwell, abide, reside (permanently)".
+Paul Strack
Александр Запрягаев Dec 13, 2016 (10:35)
To must be a preposition because tarassi is at this stage already barely possible as a locative (number markers precede all). Tarass-i can only be an ACC plural for sg. taras(s) and thus requires a preposition unless transitive, which har does not give the impression of belonging to.
Paul Strack Dec 13, 2016 (20:44)
I will make a note to fix this in the next version. Thanks for the correction.
Tamas Ferencz Dec 13, 2016 (21:36)