Theory:

     When we got into the town, I had a good wait. Master’s business kept him for a long time, so we could not start for home till late in the afternoon. By then, the wind was much stronger and I heard Master say to John that he had never been out in such a bad storm. We went through a wood where great branches were swaying about like twigs and the wind made a terrible rushing sound.

     “I wish we were out of this wood,” said my master.
 
     “Yes, sir,” replied John. “I hope no branches fall down on us,” The words were hardly out of his mouth when there was a groan and a splitting sound. Then a Oak tree that had been torn up by the roots came crashing down and fell right across the road in front of us.

     I will never say I was not frightened of, for I was. I stopped still trembling. Of course, I did not turn around or run away. I was not brought up like that. John jumped out and in a moment was standing by my head. He held my bridle and calmed my nerves.
Explanation:
  
After they reached the town, the horse had to wait for a long time. His master's work kept him busy for a long time, and they could not start their return journey almost till evening. The weather had worsened by that time. The winds were stronger, and the master had told John that they had never travelled in such a bad storm. They passed through the woods where the branches were swinging and shaking like small twigs. The winds were so strong that it shook the branches easily like it shook small twigs! The wind also had a frightening loud sound.
 
stormy woods.jpg
The stormy woods that evening, it was scary!
  
The master said he wanted to get out of the woods. John replied yes, and he hoped that branches of the trees do not fall on them, as the winds shook the branches terribly. Just as John had finished saying that, there was a huge sound of a tree breaking down. An oak tree had been uprooted and crashed down just in front of them.
 
tree uprooted.jpg
Branches of the oak tree crashed down right in front of them!
 
The horse got scared now, and he admitted it. He stopped the cart and stood there, shaking out of fear. But he did not give up and run away. He was taught to face obstacles and was grown as a brave horse. John had jumped out of the cart and stood beside the horse. He held the horse's neck straps and pacified him because John knew the horse was tensed.
 
Meanings of difficult words:
  
No.
Words
Meanings
1
groana deep sound, especially due to some damage
2
bridlea set of leather straps tied around the neck of a horse to control it
3
calmed my nervesto stop from getting nervous, to pacify
4
tremblingshaking, due to fear/anxiety
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-8. When Instinct Works - Anna Sewell(pp. 145-151). Published by the Tamil Nadu Textbook and Educational Services Corporation.