quen.

Headword: 
quen.
Principal English Translation: 

how?; what?; in what way? in what manner? (but beware not to confuse a plural ending for the preterite perfect, when it has an intrusive n)

IPAspelling: 
keːn
Alonso de Molina: 

de que manera, o como. s. acaecio esso? Aduerbio.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, f. 88v.

Frances Karttunen: 

QUĒN how? in what manner? / ¿de que manera? o ¿como? (M).
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 209.

Horacio Carochi / English: 

quēn = how
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 510.

Andrés de Olmos: 

Quenin, quen? como, o en que manera?
Andrés de Olmos, Arte para aprender la lengua Mexicana, ed. Rémi Siméon, facsimile edition ed. Miguel León-Portilla (Guadalajara: Edmundo Aviña Levy, 1972). 185.

Attestations from sources in English: 

Yhuan quen ca tle yca tle ypampa = And how, why, for what reason (central Mexico, early seventeenth century)
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 2, 216–217.

Auh quen ca tle yca tle ypampa = And how, why, for what reason (central Mexico, early seventeenth century)
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 2, 216–217.

Quēn in? or Quēnin in? = How so?
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 132.

quen cate. quen nemi. yn incah. yn culhuaque Aco quintollinia = How are they? How do they live with the Culhuaque? Are [the Culhuaque] harming them? (central Mexico, early seventeenth century)
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 1, 112–113.

When preceded by yn (in), it loses the question mark.
Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.

quẽ ҫan nel oc nen = What can be done? Is it yet in vain? (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 84.

quen mach huel = How fortunate! (central Mexico, sixteenth century)
Personal communication, James Lockhart, in sessions analyzing Huehuetlatolli.

quen nel = What can be done? (central Mexico, sixteenth century)
Personal communication, James Lockhart, in sessions analyzing Huehuetlatolli.

Attestations from sources in Spanish: 

ke:n = quen
¿Ke:n tine:mi? Yek...¿ua:n usted? = ¿Co:mo esta:s? Bien....¿y usted? (Sonsonate, El Salvador, Nahuat or Pipil, s. XX)
Tirso Canales, Nahuat (San Salvador: Universidad de El Salvador, Editorial Universitaria, 1996), 21–22.