The phrase "is case" is not a standalone word, but it can be part of other words, such as suitcase or lowercase. Antonyms are words that have the opposite meaning of a given word. In the case of the word "is," which is a form of the verb "to be," its opposite would be "is not" or "isn't." For example, the antonym for "positive case" could be "negative case," and the antonym for "uppercase" could be "lowercase." Understanding antonyms is an important aspect of building a strong vocabulary and improving language skills, as it helps us communicate more clearly and effectively.