John McWhorter's "Words on the Move" argues that language is perpetually changing, unlike the static image presented by dictionaries. These changes involve both the meanings of words (semantics) and their sounds (vowel shifts). One key type of semantic change is when words become more personal and subjective, evolving into what McWhorter terms Modal Pragmatic Markers (MPMs). These MPMs, categorized by the FACE schema (Factuality, Acknowledgment, Counterexpectation, Easing), add emotional and attitudinal nuances to communication. Examples include really for emphasis, well for polite acknowledgment, and counterexpectational uses of words like ass.

The book highlights that words rarely maintain their original meanings. Shakespeare's English, for instance, uses words like reduce and awful with different connotations than today. Even seemingly fixed grammatical elements evolve, such as the contraction let's and the grammaticalization of words like ass and all. Vowel shifts, like the Great Vowel Shift and the Northern Cities Shift, are ongoing processes that alter pronunciation over time. Compounding, the combining of words, is another crucial mechanism for language evolution.

McWhorter emphasizes that writing creates an illusion of linguistic stability, while spoken language is in constant flux. Modern linguistic novelties are not errors but part of this natural evolution. Ultimately, "Words on the Move" encourages a view of language change with curiosity and wonder rather than judgment, recognizing it as a fundamental aspect of human expression. The book suggests that attempting to enforce rigid linguistic standards overlooks the dynamic and ever-moving nature of language.