觉得 (jué de) means 'to think' or 'to feel' based on your own opinion or impression — it's less formal and less certain than 认为 (rèn wéi, 'to believe/consider'). Use 觉得 for personal opinions (觉得这本书好) and for physical feelings (觉得冷); for specific emotions like 'happy' or 'sad', 感到 (gǎn dào) is more natural.