When do I use 确定 versus 决定？
确定 means 'be sure' or 'comfirm'/'be confirmed'. 我确定他会回来的.(I'm sure he will be back.)

决定 (v) means 'decide to do'. 我决定做...(I decide to do something...)

决定 (n) means 'decision'. 我终于做出了决定.(Finally, I made my decision.)