Look at the following set of sentences. We may all agree that both mean the same. But the difference here is in the function of the words such as his and my in the first set of sentences which are possessive adjectives and the use of words his and mine in the second set which are possessive pronouns. So what you have to learn basically is the difference between a possessive pronoun and a possessive adjective.
To understand the difference between a possessive pronoun and a possessive adjective, it is important to know the function of a pronoun and adjective as parts of speech. A pronoun is a word that takes the place of a noun in a sentence. It is common to replace the noun with a pronoun when it is used repeatedly in a sentence or paragraph. Thus while the name is used in the introductory sentence, it is often replaced by pronouns such as he, she, him or her in the consecutive sentences. For example, in the sentence, "Tom is an intelligent boy. He always scores good grades in his assignments." The pronoun ‘he’ is used instead of repeating the name ‘Tom’.
Possessive is used to show possession, when something is owned, thus we can have possessive nouns, possessive adjectives and possessive pronouns. For example in the sentence “Sam’s son bought a new car” Sam is possessive as he has a son. ‘Sam’s’ is a possessive noun. If you replace ‘Sam’s with ‘his’ you would be using the possessive adjective- “His son bought a new car’. ‘His’ is a possessive adjective as it is showing possession and also describing the son. The same sentence could be used with a possessive pronoun as in “The car is his” which means the car belongs to him. Thus if the possessive word describes a noun it is called a possessive adjective and when the possessive word replaces a noun it is called a possessive pronoun.
Possessive pronouns replace nouns that show ownership or possession. A list of possessive pronouns would include the words, mine, his, hers, yours, its, ours, theirs.
Possessive adjectives modify nouns that show ownership or possession and usually placed before a noun. A list of possessive adjectives would include the words, my, his, hers, yours, its, our, their.
The rules in English grammar may often be complex and confusing but the key to using and speaking correctly in English is to get a hang of the grammatical rules and use them confidently in communication.