Being substantive means being significant, meaningful or having value. Its antonyms could be trivial, insignificant, meaningless, unimportant, or frivolous. When something is not substantive, it lacks substance or importance. It could also be considered as shallow, superficial, or irrelevant. Being substantive often refers to ideas, arguments, or opinions that carry weight or have impact. Antonyms for substantive ideas might include insignificant, unconvincing, or flimsy. When writing or speaking, it is important to choose words that can effectively convey the intended message, and knowing the antonyms for substantive can help clarify the opposite meaning.