Grammatical categories are classifications that help us understand how words function in a sentence. Common categories include tense, number, gender, and case. These categories provide rules for how words change form to convey different meanings, such as whether an action is happening in the past, present, or future. Each language has its own set of grammatical categories, which can vary widely. For example, in English, nouns can be singular or plural, while in Spanish, nouns have gender, being either masculine or feminine. Understanding these categories is essential for constructing grammatically correct sentences.