The antonyms for "were careless" would be "were cautious," "were attentive," "were diligent," "were careful," or "were vigilant." These words signify the opposite of carelessness, which is a lack of attention, focus, or concern. They suggest that the person or people in question were responsible, attentive, and conscientious in their actions or behavior. Being careful and cautious can prevent accidents or mistakes from happening, and being diligent and attentive can ensure that things are done correctly and efficiently. Therefore, it is important to be mindful of our actions and strive to be careful, attentive, and diligent in all that we do.