John has sent a book to Rahul. He is a good teacher.
Here I don't know who is good teacher John or Rahul. The he Introduc to who. Could you explain me that Rahul is good teacher because the pronoun is nearest than John. Or John is good teacher because he is subject pronoun.
The expression is ambiguous. Both interpretations are possible.
Let's think of the meaning. Rahul received a book; does this make him a good teacher? It seems unlikely. You can send a book to a good teacher or to a bad teacher. On the other hand, John sent a book; this is the sort of thing that good teachers do. Good teachers send books to help their students.
Grammatically it is ambiguous. Pragmatically there is only one likely interpretation: John is the teacher. When reading, think about how the meanings fit together.
To add to the correct answer by James K, if you wanted to say that Rahul is a good teacher without repeating the name Rahul, you could say:
John has sent a book to Rahul, who is a good teacher.
Rahul is the good teacher.
If John is the good teacher:
John, who is a good teacher, sent a book to Rahul.
And sometimes we will use the phrase "the latter" to refer to the second person mentioned:
John has sent a book to Rahul. The latter is a good teacher.
Rahul is the good teacher.
