There are subject and object pronouns. They can replace people or things.
When we talk about people
||I call Mary. → Mary calls me.
||You call Mary. → Mary calls you.
||He calls Mary. → Mary calls him.
||She calls Mary. → Mary calls her.
||We call Mary. → Mary calls us.
||They call Mary. → Mary calls them.
We always write the pronoun I with a capital letter.
When we talk about things
When we talk about things, plants, animals, babies, we use "it" for the singular and "they" for the plural.
— I like ice-cream.
— It is tasty. I like it, too. WRONG He is tasty. I like him.
I have a bicycle. It is nice. I ride it very often. WRONG He is nice. I ride him.
I watch a lot of films. They are interesting.
I have a lot of books. They are useful. I need them.