Theory:

“Alas! I have no ruby now,” said the Prince. “My eyes are all that I have left. They are made of rare sapphires, which were brought out of India a thousand years ago.” He ordered the swallow to pluck out one of them and take it to the playwright. “He will sell it to the jeweller, and buy firewood, and finish his play,” he said.

“Dear Prince,” said the swallow, “I cannot do that,” and he began to weep.

“Swallow, Swallow, little Swallow,” said the Prince, “do as I command you.”

So the swallow plucked out the Prince’s eye, and flew away to the young man’s garret. It was easy enough to get in, as there was a hole in the roof. Through this he darted, and came into the room. The young man had his head buried in his hands, so he did not hear the flutter of the bird’s wings, and when he looked up he found the beautiful sapphire lying on the withered violets.

“I am beginning to be appreciated,” he cried. “This is from some great admirer. Now I can finish my play,” and he looked quite happy. The next day the swallow flew down to the harbour. He sat on the mast of a large vessel and watched the sailors working. “I am going to Egypt,” cried the swallow, but nobody minded, and when the moon rose he flew back to the Happy Prince.
Explanation:
 
Later, the prince exclaimed that he had no ruby with him. He had already given it to the poor woman. Then he told the swallow that what he had was only the two eyes. Those two eyes were made of precious and rare sapphires. The sapphires were brought from outside India thousands of years ago. He ordered the swallow to pluck one of the eyes and give it to the young man. Then he said that the man would sell the sapphire to the jeweller and buy firewood woods, and he would finish the play. As a result, Happy Prince instructed the swallow to take the sapphire from one of his eyes and present it to the young man. When the swallow learned that the Happy Prince wished to donate one of his eyes to the young man, he began to mourn. He informed Happy Prince that he was unable to do so.
 
On the other hand, the Happy Prince told the swallow to do what he had told him to do. Finally, the swallow plucked out one of the eyes of the Happy Prince and flew to the young man's garret. The roof of the room had a hole in it. As a result, the swallow swooped into the room via that gap. The youngster was sitting with his head buried in his hands, and so he didn't hear the bird's wing flutter. Later, when the man looked up, he saw the beautiful sapphire lying on the dried petals.

When the young man saw the beautiful stone, he thought that some admirer, or some person who praised him, had sent him a gift. So, he thought that with this, he could finish his play. It means that the sapphire would help him to get rid of his hunger. The following day, the swallow flew down to the harbour and sat on a mast of a ship. He spoke loudly that he was going to Egypt. But nobody minded the swallow because the human beings did not know the good deeds done by the sorrow. And when it was night, he flew back to Happy Prince.
 
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A swallow in the harbour
 
Meanings of the difficult words:
  
S.No
Words
Meanings
1
JewellerA person or company that makes or sells jewels or jewellery
2
GarretA very small, uncomfortable room at the top of a house
3
FirewoodWood that is burnt as fuel
4
AdmirerSomeone who admires someone or something
5
SailorA person whose job it is to work as a member of the crew of a commercial or naval ship or boat, especially one who is below the rank of officer
6
DartMove or run somewhere suddenly or rapidly
7
Flutter Of a bird or other winged creature fly unsteadily or hover by flapping the wings quickly and lightly
Reference:
National Council of Educational Research and Training (2006). Moments. The Happy Prince – Oscar Wilde (pp. 28-35). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.