A polysynthetic language is characterized usually by verbal inflections to the point that word order is largely free. Natlangs that are polysynthetic include Mohawk. It is common in North American Indian languages. There are few polysynthetic conlangs, like Bp@x’àãókxá, Noyatowa, Ilothwii and Terpish.
A polysynthetic language generally marks the verb for the number, person, and class (if there are noun classes) of the subject, and sometimes the direct object. The verb is generally marked through affixes that may be prefixes or suffixes. The language may be agglutinative or fusional. In addition, tense, aspect, and any other information about the verb are usually marked on the verb.
Here's a hypothetical example:
|He had seen them.
In addition, it is frequent for polysynthetic languages to incorporate nouns. This means that the noun forms a compound with the verb. To illustrate again:
|He had seen rocks.
A great deal of explanation can be found here:
Jeff Burke has written this description of polysynthetic languages:
Polysynthesis is a grammatical phenomenon most famously present in many North American Indian languages; the Algonquian and Iroquoian tongues are the most well-known. These languages, instead of using case endings on nouns and conjugational endings on verbs, mark both subject and object inside the verb; further, subject and object inflections are considered the actual arguments of the verb, while any free-standing nouns associated with a verb are like dislocated topic phrases. Here is an example from Mohawk, an Iroquoian language spoken in upstate New York and southern Ontario:
Nominalized and rendered in normal English, this is 'I-ate a-banana'; but a more literal translation would be 'A-banana, I-ate-it'. The pronominals 'I' and 'it' (the arguments of the verb root -k- 'eat') are expressed via the prefix ke-, which indicates a singular first-person subject and a singular third-person object. ke- is, then, akin to noun case and verb conjugation fused into a single marker and attached to the verb. Mohawk has some fifty-odd of these pronominal prefixes that express subject-object combinations; and a prefix is mandatory on every verb. By contrast, the noun teiotahià:kton 'banana' stands in relation to the verb wá:keke’ only indirectly, as a topical phrase stands in relation to a central clause in English.
Cheyenne, an Algonquian language spoken in southern Montana and Oklahoma, also marks subject and object on the verb, but does so differently; instead of a fusional marker, it uses a prefix for subject and a suffix for object. Witness:
Literally translated, this means 'A-man, I-saw-him'. Hetane is the noun 'man' standing in relation to the verb návóómo 'I-saw-him' like a topical phrase. Ná- is the prefix for a singular first-person subject; -o is the marker for a singular third-person object.
In addition to marking both subject and object inside the verb, polysynthetic languages often make use of noun incorporation. This is a phenomenon where a noun becomes, in essence, an inflection on a verb. Incorporated nouns can be, depending on the language and circumstances, direct objects, indirect objects, subjects, locatives, and of other kinds. Mohawk makes heavy use of incorporation, as in:
This translates as 'It-is-a-good-shirt', where the noun root atia’tawi (a word that can be used to refer to basically any upper-body garment) is present inside the verb. Cheyenne also uses noun incorporation on a regular basis:
This verb, meaning 'I-have-a-big-house', contains the noun morpheme maheo 'house'.
Noun incorporation can look bewildering, but is best understood in terms of English compounding. Babysit is an example of a word with an incorporated direct object; and words like deer-hunter and manslaughter make use of the same kind of incorporation.
The headmarking inflections can be better understood by thinking of Romance languages like Spanish, which inflects for details about the subject. Polysynthetic languages take this concept further and inflect the verb for details about the object as well.