-tla-.

Headword: 
-tla-.
Principal English Translation: 

indefinite nonpersonal obj.; prefix of verbs; also prefixed to relational words to make absolute forms; also prefixed to intransitive verbs to make impersonals

& xi- (imperative) = "please"; a polite way of phrasing the imperative

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), 235.

Attestations from sources in English: 

Celiya in cuahuitl = The tree turns green, Tlaceliya = Everything turns green
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 141.

-Tla- is an indefinite prefix that is used to refer to non-humans, indefinite things or animals. E.g. nitlacaqui = I hear things, or I hear something.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 29.

In Classical we have "tlate:ntli" with two definitions: 1. cosa echada (which comes from te:ma) and 2. cuff (of a shirt of pair of pants) (from te:ntli). They are obviously two different words. In Modern Huastecan Nahuatl, for "tlate:ntli" we have 1. "at the side of the road where the forest begins" and 2. "a sharp knife or machete." Both of these come from "te:ntli". I'm interested in the "tlate:ntli" that comes from "te:ntli." What is the "tla-" doing?
In Modern Huastecan Nahuatl we have "tlaye:ctli," meaning "something that no longer has little specks of things (basuritas) mixed in with it." It looks like this is built on a noun, "ye:ctli", in the same way that "tlate:ntli" is. Again, what is the "tla-" doing?
Consider these examples, in which final morphemes are deleted in the process of converting a verb to a noun, and how parallel options are used to provide shades of meaning:
1. cuelpachoa, nic, "to fold s.t." > tlacuelpachtli, "a cuff" vs tlacuelpacholli, "folded clothing."
2. payania, nic, "to crumble s.t." > cintli tlapayantli, "corn pieces" vs "cintli tlapayanilli", "crumbled corn."
3. tentia, nic, "to sharpen s.t." > tlatentli, "cochiyoh tlatentli", "a sharp knife," vs. "cochiyoh tlatentilli", "a sharpened knife."
4. pahtia, nic. "to fertilize or apply herbicide to a plant." > "toctli tlapahtli", "a fertilized corn plant," "toctli tlapahtilli," "a corn plant that has been fertilized."
On the one hand we have objects that have undergone a completed action, and on the other hand we have words that emphasize more the process which the objects have undergone. Then there that little detail of the morpheme deletion.
John Sullivan, two Nahuat-L listserv postings, March 7, 2012.