The word "begun" signifies the start or initiation of something. There are numerous antonyms for this word that can be used depending on the context. Among the most commonly used antonyms are "end," "conclude," "finish," and "terminate," which represent the opposite action to that of beginning. Other antonyms for "begun" include "halt," "cease," "stop," "oppose," "discontinue," "suspend," and "abandon." These words are used to describe the opposite of the act of starting, and they convey the message of coming to an end or stopping something. Depending on the context, you can choose the right antonym to convey your message more accurately.