A pronoun takes the place of a noun.

Example story:

Mary is one of the heads of the ToJi Corporation. Mary works with Mr. James and Mr. James' son Tom. Mr. James and Mr. James' son Tom are experts in biochemistry. Mary, Mr. James, and Tom researched and invented a drug for cancer treatment.

If the story above is written using pronouns:

Mary is one of the heads of the ToJi Corporation. She works with Mr. James and his son Tom. He and his son Tom are experts in biochemistry. They researched and invented a drug for cancer treatment.
Personal Pronouns

Personal pronouns refer to a person:

I go to school.
You are a student.
They are Koreans.
He works here.
We gave her food.
The word ‘it' refers to an object:

I drank it.
It is big.
They cut it into halves.
Memorize the personal pronouns: