What is the main idea of story ugly duckling?

The main idea of Hans Christian Andersen's "The Ugly Duckling" is that inner beauty and worth are far more important than outward appearances. The story shows how appearances can be deceiving and how judgment based solely on looks can lead to misunderstanding and unhappiness. The duckling's eventual transformation into a beautiful swan highlights the idea of self-acceptance and the importance of believing in oneself, even when others don't understand or appreciate you.