The word "are substantive" refers to something that is substantial, important or meaningful. Antonyms for this word could include terms such as insignificant, trivial, unimportant, or lightweight. These are terms that indicate something lacks substance, value or significance. Another antonym might be superficial, meaning that it is only surface level and lacks depth. These antonyms are useful when describing things like ideas, arguments or opinions that are not well-founded or do not carry weight. By using antonyms for "are substantive," speakers and writers can more clearly communicate when something lacks substance or value.