Morphological rules are guidelines that govern how words are formed and structured in a language. They involve the combination of prefixes, suffixes, and root words to create new meanings. For example, adding the suffix “-ed” to the verb “walk” forms the past tense “walked.” These rules help in understanding how different forms of a word relate to each other. For instance, the root word “happy” can change to “happiness” by adding the suffix “-ness.” This process allows speakers to expand their vocabulary and express a variety of ideas using a limited set of root words.