Theory:

     Then he saw a man in a shop buying a newspaper. The man’s umbrella stood beside the door. Soapy stepped inside the shop, took the umbrella, and walked slowly away. The man followed him quickly.

     “My umbrella,” he said.

     “Oh, is it?” said Soapy. “Why don’t you call a cop? I took it. Your umbrella! Why don’t you call a cop? There’s one standing at the corner.”
 
     The man walked more slowly. Soapy did the same. But he had a feeling that he was going to fail again. The cop looked at the two men.
Explanation:
 
The next scene takes place in front of a shop. Soapy then noticed a man in the shop buying a newspaper. The man kept his umbrella beside the door. On seeing the umbrella, Soapy thought of a plan. So he entered the shop and took the umbrella and walked slowly away from there.
 
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A man buying a newspaper in a shop

But when the man noticed Soapy stealing his umbrella, he followed him. The man approached Soapy and said that the umbrella was his. Soapy immediately answered that if it was his umbrella, he could report it to the cop. Soapy also acknowledged that he had taken the umbrella and asked, “Why don’t you call a cop?” Soapy’s intention was to make the man angry and so he could complain to the cop. Soapy also told him that a cop was standing at the corner. This was the another way of conveying him to call a cop.

When the umbrella man saw the cop, he walked very slowly. Soapy also walked slowly as the man did because he was worried that whether his plan would fail again. Soapy had the inner feeling that he was about to fail once more. However, a cop was standing nearby and watched the activities of the two men.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Umbrella A device consisting of a circular canopy of cloth on a folding metal frame supported by a central rod, used as protection against rain
2
CornerA place or angle where two sides or edges meet
3
Feeling An idea or belief, especially a vague or irrational one
Reference:
National Council of Educational Research and Training (2007). An Alien Hand. The Cop and the Anthem- O. Henry(20-29). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.