Proto-Nahuan is the hypothetical daughter language of the Proto-Uto-Aztecan language

Proto-Uto-Aztecan language

The Proto–Uto-Aztecan language is the hypothetical common ancestor of the Uto-Aztecan languages.-Vowels:Proto–Uto-Aztecan is reconstructed as having an unusual five-vowel system:...

 which is the common ancestor from which the modern Nahuan languages

Nahuatl

Nahuatl is thought to mean "a good, clear sound" This language name has several spellings, among them náhuatl , Naoatl, Nauatl, Nahuatl, Nawatl. In a back formation from the name of the language, the ethnic group of Nahuatl speakers are called Nahua...

 have developed. Some phonological changes shared by all Nahuan languages are:

Proto-Uto-Aztecan *t becomes Proto-Nahuan lateral affricate *t͡ɬ before Proto-Uto-Aztecan *a ( PUA *ta:ka "man" becomes PN *tla:ka-tla "man" ) Proto-Uto-Aztecan initial *p is lost in Proto-Nahuan. (PUA *pahi "water" becomes PN *a:-tla "water") Proto-Uto-Aztecan *u merges with *i into Proto-Nahuan *i (PUA *muki "to die" becomes PN *miki "to die) Proto-Uto-Aztecan sibilants *ts and *s split into *ts, *ch and *s, *ʃ respectively. Proto-Uto-Aztecan fifth vowel reconstructed as *ɨ or *ə merged with *e into Proto-Nahuan *e (PUA *nɨmi "to walk" becomes PN *nemi "to live, to walk") a large number of metatheses Metathesis (linguistics) Metathesis is the re-arranging of sounds or syllables in a word, or of words in a sentence. Most commonly it refers to the switching of two or more contiguous sounds, known as adjacent metathesis or local metathesis:...  in which Proto-Uto-Aztecan roots of the shape *CVCV have become *VCCV. (PUA *pu:li "to tie" becomes PN *ilpi "to tie" )

Morphology

The hypothetical Proto-Nahuan is hypothetically an agglutinative language, where words use suffix complexes for a variety of purposes with several morpheme

Morpheme

In linguistics, a morpheme is the smallest semantically meaningful unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word,...

s strung together.