The processes used are clisis, reduplication, ablaut, compounding, suppletion, order and the most common one is affixation (suffixation in particular).
en.wikipedia.org Reduplication has a grammatical function in some languages, such as plurality or intensification.
en.wikipedia.org A more complicated and speculative form of source criticism results from critical evaluation of style, vocabulary, reduplication, and discrepancies.
en.wikipedia.org On the contrary to the above definition, there are some words in some of the languages in which reduplication has the opposite meaning.
en.wikipedia.org Exponents used in verb conjugation include prefixes, suffixes, infixes, and reduplication.
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