A prefix is an affix that is placed before the stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix un- is added to the word happy, it creates the word unhappy.

English Grammar Terms I

In linguistics, the grammar of a natural language is its set of structural constraints on speakers 'or writers' composition of clauses, phrases, and words. The term can also refer to the study of such constraints, a field that includes domains such as phonology, morphology, and syntax, often complemented by phonetics, semantics, and pragmatics. There are currently two different approaches to the study of grammar, traditional grammar and theoretical grammar.