商量 (shāng liang) is the everyday, conversational verb for 'to discuss' or 'to talk over' — it implies a mutual exchange of opinions to reach a decision, like discussing plans or a problem with friends. 讨论 (tǎo lùn) is more formal and often used for academic or structured debates; 商讨 (shāng tǎo) is even more formal, used in business or official settings.