1. This (pl. These) - pronoun and determiner is used with the object/person that
(i) is new -> Introduction: “This is my sister.”
(ii) the first thing -> “This first example is very valuable”
(iii) is being experienced presently and ‘for the first time’ -> “This is my first time in Africa.”
(iv) is the only thing being presently discussed/experienced -> If you have described the main idea, concept, thing, person, then: "This could be worth a fortune.” “Nowadays, a lot of young women want to have plastic surgery. This has become a social issue in my country.
(v) will be immediately discussed/experienced, -> “I would like some more information on this idea of yours.”
(vi) Is the nearest thing (in space or time) -> “This ring in my hand is very expensive.”
(vii) Is the thing now indicated -> “Moving on, if I may show you this. [points at something]…”
2. That (pl. Those) pronoun and determiner is the object/person that
(i) has been discussed/experienced, -> “I told you about that yesterday!”
(ii) is the next and subsequent thing, -> “This pin is rusty, now let’s look at that one.
(iii) was experienced in the past -> “I went to Zambia in 1987. That was my first time in Africa.”
(iv) is the second or subsequent thing being discussed/experienced -> You have told me about your experience in Africa, but you also went to America, tell me about that.
(v) a more distant object in space or time-> “That over there is my shopping list.” / “Ah yes, 1950… that was the year I went to Brazil.”