There are several synonyms for the word "couldn't" that can be used to convey a similar meaning. Some of the most commonly used synonyms for "couldn't" include "unable to," "incapable of," "powerless to," and "not able to." These words all convey a sense of being unable to accomplish something or lacking the ability to do so. Other synonyms for "couldn't" include "failed to," "did not manage to," and "wasn't able to," which imply a similar sense of inability. In using these synonyms, writers and speakers can vary their language and express their ideas in different ways while still conveying the same basic meaning.