हि

Monier Williams (1899) “हि”, in A Sanskrit–English Dictionary, […], new edition, Oxford: At the Clarendon Press, →OCLC, page 1297,1303.
William Dwight Whitney, 1885, The Roots, Verb-forms, and Primary Derivatives of the Sanskrit Language, Leipzig: Breitkopf and Härtel, page 205
Otto Böhtlingk, Richard Schmidt (1879-1928) “हि”, in Walter Slaje, Jürgen Hanneder, Paul Molitor, Jörg Ritter, editors, Nachtragswörterbuch des Sanskrit [Dictionary of Sanskrit with supplements] (in German), Halle-Wittenberg: Martin-Luther-Universität, published 2016
Arthur Anthony Macdonell (1893) “हि”, in A practical Sanskrit dictionary with transliteration, accentuation, and etymological analysis throughout, London: Oxford University Press

Mallory, J. P. with Adams, D. Q. (2006) The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World (Oxford Linguistics), New York: Oxford University Press, →ISBN, page 245
Rix, Helmut, editor (2001), Lexikon der indogermanischen Verben [Lexicon of Indo-European Verbs] (in German), 2nd edition, Wiesbaden: Dr. Ludwig Reichert Verlag, →ISBN, page 174
Pokorny, Julius (1959) chapter 0424, in Indogermanisches etymologisches Wörterbuch [Indo-European Etymological Dictionary] (in German), volume 2, Bern, München: Francke Verlag, page 0424

हि • (hí)

for, because, on account of (never standing first in a sentence, but generally after the first word and used enclitically, sometimes after pronouns)
सर्वो हि पृतना जिगीषति (sárvo hí pṛ́tanā jígīṣati) — for everybody wishes to win battles

भवान् हि प्रमाणम् (bhavān hi pramāṇam) — for your honour is the authority

तथा हि (tathā hi) — for example; accordingly

न हि (ná hí), नहि (nahí) — for not; not at all
just, pray, do (with an imperative or potential emphatically; sometimes with indicative)
पश्यामो हि (paśyāmo hi) — we will just see
indeed, assuredly, surely, of course, certainly
हि वै (hí vaí) — most assuredly

हि – तु (hi – tu), हि – पुनर् (hi – punar) — indeed – but
often a mere expletive, especially to avoid a hiatus, sometimes repeated in the same sentence; hi is also said to be an interjection of "envy", "contempt", "hurry" etc.

Monier Williams (1899) “हि”, in A Sanskrit–English Dictionary, […], new edition, Oxford: At the Clarendon Press, →OCLC, page 1297/3.
Mayrhofer, Manfred (1996) Etymologisches Wörterbuch des Altindoarischen [Etymological Dictionary of Old Indo-Aryan]‎^[1] (in German), volume 2, Heidelberg: Carl Winter Universitätsverlag, pages 814-5