We use the subject pronouns to substitute the person already  mentioned.

The subject pronouns are used at the beginning of the sentence

Example:     

         Jennifer is 20 years old. She is from Canada.

         Roberto is a nice person. He is my friend.

         My house is a residence. It is in Cancun.

Ann and I are 16 years old. We are in the same school.

Those people  are not from Mexico. They are americans..