Subject pronouns are used as grammatical subjects in a sentence.
A subject pronoun does the action of the sentence instead of receiving the action, as an object does.
In English, sentences must begin with the subject. If the subject is not determined by some appellation or a name, then personal pronoun must be used:
I won’t be late.
You remind me of my brother.
He left five minutes ago.
We came back on Monday.
An object pronoun receives the action instead of doing the action itself. They are contrasted with subject pronouns.
This type of pronoun always stands after a verb:
I love her.
I haven’t seen them.
The driver asked him where to go.
If a preposition appears directly after a verb then a personal pronoun stands right after a preposition:
The girl smiled at us.
Please, don’t go without me.
Possessive adjectives called “possessive pronouns” determine the affiliation of something or relation with somebody.
Possessive adjective always appears before determining the noun.
You cannot use possessive adjective without a noun:
My sister is older than me.
Her father was a famous musician.
It was an old table. Its legs were broken.
These are our tickets.