Morphological rules are guidelines that govern how words are formed and structured in a language. They involve the study of morphemes, which are the smallest units of meaning, such as prefixes, suffixes, and root words. These rules help in understanding how different forms of a word can be created, like changing a verb into a noun or altering a word to indicate tense. For example, in English, the morphological rule for pluralization often involves adding the suffix -s to a noun, as in cat becoming cats. Similarly, the rule for forming the past tense typically involves adding -ed to regular verbs, such as walk becoming walked. These rules are essential for constructing grammatically correct sentences.