Great question! “This” and “it” are different types of pronouns, and there’s a lot to say about them. I’ll give you some basic tips to keep in mind.
“It” is more general. It can be a subject pronoun or an object pronoun. You can use it to speak generally about a thing that is either physically present, not present, or abstract.
“This” is more specific. It is a demonstrative pronoun (“that,” “those,” and “these” are also demonstrative pronouns). I think of these pronouns as “pointer pronouns” because we use them when we can also point our finger at the object that we are talking about. Although there are some situations in which you can use “this” abstractly or for things that are not present, we use it most often when something is physically present.
You can think about these examples:
- I love the book A Tale of Two Cities. It is a great book! (The book doesn’t need to be present.)
- Is this your copy of A Tale of Two Cities? This is a great book. (The book is near the speaker.)