First, put your fingers on the keys. Fingers must stand up, or you can raise your palm properly. Strength is concentrated at the fingertips like a yurt, and the upper part is slightly raised. Hold your hands and move your arms outward (that is, open your hands after moving).
Grasp the opening angle by yourself. In fact, it is better to bend a little when playing the piano, because playing the piano with your arms alone will definitely be very tiring, so you should combine your strength with your waist. Your teacher let you fall freely (so did mine).
At first, it will be uncomfortable. If beginners are generally young, teachers will not pay so much attention to the placement of arms. This problem will appear in the future, and we must find ways to correct it.
2. Mentality problem
In the process of playing, it is inevitable to encounter some difficulties. When you practice for a long time or are unfamiliar with it, you are particularly nervous and your arms will become stiff. Or sometimes pay special attention to whether the arm is tight, but I don't feel it. Treating it with a normal heart may solve the problem.