A prefix is a bound morpheme that attaches to the beginning of the stem of a word to form either a new word or a new form of the same word.
Prefixes are types of affixes.
All prefixes in English are derivational, meaning the affixes create new words.
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Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.