Antonyms for the word "not main" include "principal," "central," "primary," "mainstream," "dominant," "chief," "foremost," "leading," "essential," "significant," and "major." These terms convey the importance and prominence of something or someone, indicating that they are the primary focus or center of attention. In contrast, "not main" suggests a secondary or peripheral role, with less significance or impact. When choosing the right antonym for "not main," it's essential to consider the context and meaning of the sentence to ensure that the right word is being used to convey the intended message.