The antonyms for the phrase "was careless" include being diligent, attentive, responsible, diligent or cautious. Being diligent means being careful and persistent in your work or duties. Being attentive means paying close attention to details, being alert and observant. Being responsible means being accountable for one's actions, making sure to meet expectations and obligations. Being cautious means being careful and avoiding unnecessary risks. These qualities are important in many situations, such as in the workplace, while driving, or in relationships with others. By being diligent, attentive, responsible, and cautious, one can avoid careless mistakes and achieve success in various aspects of life.