Usually "it" is a pronoun used in verb conjugations.
eg. I have, you have, he, she or it has,
I am, you are, he, she or it is,
I give you give he, she or it gives.
(In conversation "it is" can be abbreviated to "it's". The apostophe ' is put in to denote a contraction or shortened form. Note though that there is a word "its" without an apostophe. This is the possessive form of the pronoun. eg. The dog is large. Its colour is grey.)
But you might be after the difference between "it" and "this" and when to use these.
"THIS" is a demonstrative pronoun and adjective. Like if you were holding a pen, you could say "
this pen works well"
You could say "I have a pen,
it works well". The difference being in this sentence you are not indicating which pen. You are just saying you have a pen that works well.
Whereas, in "
this pen works well" you need to be demonstrating, like holding up a pen or pointing to the pen or similar. When you are in a situation where somone else trying to write with a bad pen, you could also just say "
this works well" and hand it to them. It is obvious the "this" is the pen you are holding.
With "
this" the person or thing being referred to, needs to be close by, just mentioned or about to be mentioned.
An example of
this mentioned part, is exactly
this phrase. It is obvious that "this" is referring to the line before.
Another example of "it". I have a car. It goes well. But to confuse things. Someone says" where is the key" and you say "here it is". And you could be holding it up, just like for when we use "this" but we do not use it that way. I think the word "here" preceding "it" changes it.