in.

Headword: 
in.
Principal English Translation: 

this; or, he who, those who; that which

Orthographic Variants: 
yn
IPAspelling: 
iːn
Alonso de Molina: 

In. sirue de honrato enesta lengua, y en composicion significa, los que. exemplo intlaqua. quiere dezir los que come˜. &c.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, f. 38r.

Frances Karttunen: 

ĪN proximal particle this, these / ésto (C) This is often postposed after the element it refers to and becomes bound to it as a suffix, YEHHUĀTLĪN ‘this one, this person,’ INĪN ‘this’, IMMANĪN ‘at this very time.’ It contrasts with the distal particle ŌN ‘that, those.’ The phrase IN IHQUEH ĪN is used for the plural.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 106.

Horacio Carochi / English: 

īn = this
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), 503.

Andrés de Olmos: 

"Esta particula por la mayor parte no significa nada en platica mas de solo ornato, aunque algunas vezes parece estar en lugar destas particulas que en Castilla dezimos : el, la , le , lo, las, les, los; otras vezes es aduerbio en lugar de sicut. Exemplo de como es aduerbio : yn mani in calli = assi esta la casa; pero es de notar que para dezir assi es esto como esto, no lo diran por aqui, sino por este aduerbio neneuhqui; plural, neneuhque. Yn nican mani in calli, çan no yuh yn mani yehuatl, assi esta aqui la casa como aquella."
Andrés de Olmos, Arte para aprender la lengua Mexicana, ed. Rémi Siméon, facsimile edition ed. Miguel León-Portilla (Guadalajara: Edmundo Aviña Levy, 1972), 183.

Lockhart’s Nahuatl as Written: 

This is what we call an article, but it could also be a subordinator, punctuator, marker of syntactic beginnings, says James Lockhart. The vowel can be written as y or i, but usually y at the beginning of a unit, and i when inside.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 164.

particle. subordinator, "article," punctuator, many translations and often none

īn. demonstrative pronoun
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 220.

Attestations from sources in English: 

In or yn is not always translated. When yn follows another item, it has the meaning of "this" -- e.g. ipampa yn = because of this. Post-posed demonstrative. In the next example, the yn refers to those who take the action in the verb that follows.
yn tlalnamaca = those who sell the land

in = this
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 220.

ichquich in = all this
Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.

in ... in niman = after X happened or will happen, then X happened or will happen. For example:
In ōquicuâ in , niman mic = After he ate this, he died.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 134.