Theory:

The next morning Sue came and sat on Johnsy’s bed. Taking Johnsy’s hand in hers she said, “I have something to tell you. Mr Behrman died of pneumonia this morning. He was ill for only two days. The first day the janitor found him on his bed. His clothes and shoes were wet and he was shivering. He had been out in that stormy night.”
 
Then they found a ladder and a lantern still lighted lying near his bed. There were also some brushes and green and yellow paints on the floor near the ladder. “Johnsy dear,” said Sue, “look out of the window. Look at that ivy leaf. Haven’t you wondered why it doesn’t flutter when the wind blows? That’s Behrman’s masterpiece. He painted it the night the last leaf fell.”
Explanation:
 
The following day Sue came and sat beside Johnsy's bed. She took Johnsy's hand in hers and said that she had something to tell her. She said that Behrman died in the morning due to pneumonia. In addition, she said that he was ill for only two days. On the first day, the janitor had found Behrman in his bed. His shoes and clothes were wet, and he was shivering. He became wet because he had gone out during the heavy stormy night.
 
Moreover, Sue said that a ladder and a lighted lantern were lying near his bed. In addition, she said that some paintbrushes and green and yellow paint were also lying nearby the ladder. Sue asked her friend Johnsy to look at the ivy leaf. She asked Johnsy whether she knew why the leaf did not shake or move while the heavy wind blew. Then she said that it was his masterpiece. Behrman's "last leaf" was called his masterpiece because the leaf seemed real.
 
The old man had painted his masterpiece when the last leaf fell. He did it because he wanted to save the life of the girl. He knew that the last leaf was the only hope of the young girl. So he painted the leaf to save the girl, and his everlasting dream came true in the end.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Janitor A person whose job is to look after a building
2
ShiverShaking slightly and uncontrollably as a result of being cold, frightened, or excited
3
Ladder Two upright lengths of wood, metal, or rope, used for climbing up or down something
4
BrushAn object with short pieces of stiff hair, plastic, or wire attached to a base or handle, used for cleaning, arranging your hair, or painting
Reference:
National Council of Educational Research and Training (2006). Moments. The Last Leaf– O Henry (pp. 44-48). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.