How to use this/that instead of it?
Recently, I've faced with a problem. the problem is that I don't know how to use this/that. I don't mean to say that I don't know how to use them at all, not a bit. I know how to use the words as indicative pronouns but there's another way to use these words. They can be used instead of it and my point is that I don't understand the logic of using them in that case.
I often hear native speakers saying "I know that/this". When they comment something they can say at the end of their speech "That's a good video". I'm used to say using it like "I know it" or "it's a good video" therefore I don't understand the logic. Could you explain to me how to use each of them cuz this topic is really complicated.

Question

How to use this/that instead of it?
Recently, I've faced with a problem. the problem is that I don't know how to use this/that. I don't mean to say that I don't know how to use them at all, not a bit. I know how to use the words as indicative pronouns but there's another way to use these words. They can be used instead of it and my point is that I don't understand the logic of using them in that case. 
I often hear native speakers saying "I know that/this". When they comment something they can say at the end of their speech "That's a good video". I'm used to say using it like "I know it" or "it's a good video" therefore I don't understand the logic. Could you explain to me how to use each of them cuz this topic is really complicated.

Answer

Using 'that' is just a little more focus, like seeing something for the first time and pointing to it. 
Using 'it' is just commenting on it, like agreeing with someone.

It's very common to use 'that' when you see/hear something new; That's good news, That's awesome!  I'm not happy about that.

If you use 'it', it seems you are already talking about it, or agreeing: 
like this:
A:  That's awesome!   B:  Yes, it is.
So, it's not really as emphatic..

:)