It´s a shame that none of the pre-Incan civilizations nor the Incas themselves had a written language. The Incas did succeed in imposing a common spoken language, Quechua, on most of their empire, which stretched from Argentina to Colombia. But many of the advanced techniques of construction, metallurgy and agriculture that they and their predecessors developed have been lost. The little that we know with any certainty about the Incas comes from oral histories which were captured in Spanish by the conquistadors.
Because Quechua is spoken only in countries where Spanish has become the predominant language, almost all of the resources for learning the language are written in Spanish. So to learn Quechua well you must first learn Spanish. I´ve been sitting in on an introductory Quechua course at San Marcos University, and it´s a fascinating language. It has an alphabet of only 16 consonants and 3 vowels -- depending on how you count them, and who´s version of written Quechua you use. There´s no b, c, d, e, f, g, o, v, x or z. The various Spanish linguists who developed written forms of the language use slightly different notations, but all add the Spanish letters ch and ñ. Five of the consonants, ch, k, p, q and t, have three variations each. One variation has an extra puff of air when it is voiced, and another has a brief stoppage of air following the consonant. One of the big challenges is tuning your ear to hear the differences between these variations.
Verbs take different endings to distinguish person and plurality, as in Spanish, but there are fewer tenses and all verbs are regular -- no exceptions to remember! Nouns have conjugations as in Latin. The biggest difference between Quechua and other languages (at least the ones I´ve been exposed to) is the extensive use of word endings to add information content that is independent of the meaning being conveyed. For example, there are endings that mean ¨I know this to be true¨, ¨I´m referring to something that you just said¨, ¨and also¨, etc. In all there are eleven different endings, with as many as five strung together at the end of a word. But the stress (with very few exceptions) is always on the second-last syllable. So the most difficult part for me in listening to Quechua has been separating the base word from the endings. If you´re interested in knowing what the language looks like or sounds like, here´s a site that has a lot of information: www.andes.org.