This document discusses morphology, which is the study of meaningful units in language. It defines morphology and describes the key concepts of morphemes, which are the smallest units of meaning, and how they are combined to form words. Specifically, it provides examples of how morphemes representing singular/plural and tense are added to words to change their meaning. The document also defines the different types of morphemes, including free morphemes, bound morphemes, prefixes, suffixes, and allomorphs, which are variants of morphemes. Finally, it lists and describes the nine main classes of words in English.