E.g when two persons are talking together, the most common communication are 1.that's good/nice/true 2.how will you find this man/how do you know this ? IN BOTH THE SENTENCES THE SPEAKER RELATES THE PREVIOUSE MENTIONED THING/SITUATION.could you tell me when to use this and when that?
In general, “this” refers to something in the speaker’s presence or close to the speaker, while “that” refers to something farther away from the speaker. “This man” would refer to a man “over here”, whereas “that man” would refer to a man “over there”. EDIT: just saw the title - ignore this part if you already understand this use of “this”/“that” for physical objects. But notice my use of “this”- I am referring to “this part” and “this use” because my explanation is right here and I just said it- so in this case [again, “this”] it’s almost the same concept as physical proximity. “That part” might refer to something mentioned a long time ago, or farther back in a text.
But often “this” or “that” do not refer to physical objects, e.g., Person A: “I just ate lunch”; Person B: “That’s [that is] nice”. I think “that” is more idiomatic when you are talking about some abstract concept (the fact that Person A just ate lunch). “This is nice” would probably also be acceptable to say, but might sound a little stiff or detached. Same with “how do you know that” vs. “how do you know this”- the former sounds more natural to me than the latter.