Theory:

He passed over the river, and saw the lanterns hanging on the masts of the ships. At last he came to the poor woman’s house and looked in. The boy was tossing feverishly on his bed, and the mother had fallen asleep, she was so tired. In he hopped, and laid the great ruby on the table beside the woman’s thimble. Then he flew gently round the bed, fanning the boy’s forehead with his wings. “How cool I feel!” said the boy, “I must be getting better;” and he sank into a delicious slumber.

Then the swallow flew back to the Happy Prince, and told him what he had done. “It is curious,” he remarked, “but I feel quite warm now, although it is so cold.”

“That is because you have done a good action,” said the Prince. And the little swallow began to think, and then fell asleep. Thinking always made him sleepy.

When day broke he flew down to the river and had a bath. “Tonight I go to Egypt,” said the swallow, and he was in high spirits at the prospect. He visited all the monuments and sat a long time on top of the church steeple.
Explanation:
 
When the bird flew over the river, ships came along the way. The bird saw lanterns hanging on their sails. Finally, the bird reached the woman’s house and looked at the boy. The boy was suffering from fever, and so he was unable to have a pleasant sleep. As a result of it, he moved about from side to side on his bed. On the other hand, his mother had fallen asleep because she was very tired due to working for a long time.
 
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A lantern in a ship
 
Later, the swallow put the ruby stone on the table near the thimble (a small cover, usually made of metal or plastic, worn to protect the finger that pushes the needle when sewing). After keeping the ruby stone on the table, the swallow went near the boy as his mother was sleeping and no one cared for him. Then the swallow shook its wings near his head, because of which the boy felt the cool air and felt better. Soon, the boy fell asleep. Even though the bird thought that he didn’t like boys, while seeing the condition of the poor boy, the swallow helped him. With this incident, we can understand that the swallow was a kind-hearted bird.
 
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Poor boy has fallen asleep

The swallow flew back and went to the statue of Happy Prince and told him what he had done. Later, the swallow said that he felt warm though the place was cold. Then the prince said that the swallow was feeling warm because he had done a good deed. It means the swallow had helped the people in need. After listening to the Happy Prince, the swallow started thinking, and when he started thinking, he felt sleepy. The following day, the bird went to the river, took a bath, and thought he would go to Egypt that night. He was delighted because he hoped that very soon he was going to meet his friends. Later, the swallow visited all the monuments of the city, and finally, he went and sat on the high tower of the church.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
LanternA lamp with a transparent case protecting the flame or electric bulb and typically having a handle by which it may be carried or hung
2
Mast A tall upright post, spar, or other structure on a ship or boat, in sailing vessels, generally carrying a sail or sails
3
Tossing To move about from side to side or turn a lot in bed, especially because you cannot sleep
4
SlumberTo sleep
5
ThimbleA small metal or plastic cap with a closed end, worn to protect the finger and push the needle in sewing
6
Feverish In a manner that shows the symptoms of a fever
7
SteepleA church tower and spire
8
Monument A statue, building, or other structure is erected to commemorate a notable person or event
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.