1. A method of dynamically modifying a probability value associated with one or more word strings that are recognized by a speech recognizer, based on semantic values associated with the word strings, comprising the steps of: creating and storing one or more rules that define a change in one or more of the probability values when one of the semantic values matches a pre-determined semantic tag, in which the rules are based on one or more external conditions about the context in which the speech recognizer is used; determining whether one of the conditions currently is true, and if so, modifying one or more of the probability values of word strings associated with semantic values that match the tag that is associated with the condition that is true.