"Couldst it" is an outdated phrase that means "could you." It's not commonly used anymore, but it's essential to understand the antonyms of this phrase. The antonyms of "couldst it" are "can't you" or "cannot you." These terms express inability or incapacity to fulfill the requested act or demand.
Using appropriate language is crucial in communication, and it's essential to use updated, clear and concise sentences rather than old fashioned phrases such as "couldst it." Using updated language will enhance communication, and the message conveyed will be clear and understandable to everyone.