'This' v. 'that' when referencing smth just mentioned

Question

I don't know, suppose it's something like this (I made up the sentences myself):

Dolphins have a smooth, streamlined body. This/that helps them accelerate up to 34.5 mph.

Sometimes, even in formal writing, diminutives supersede official names. Examples of this/that include the name of the 39th president of the United States Jimmy Carter.

Are both pronouns acceptable? It seems so, but, you have to admit, it's kinda weird considering that they are supposed to be antonyms and, as a result, not interchangeable.

Answer 1

When you are referring to something tangible, the choice between "this" or "that" is usually about proximity:

• "This" is usually used when something is right in front of you. If your friend was standing next to you and you introduced them to somebody else, you would say "this is my friend".

• "That" is usually for something that is not immediately in front of you. If your friend was at the other side of the room and you pointed them out to somebody you would say "that is my friend".

For events in time, "this" is usually used in connection with the present:

• "This is a bad year for me".

When something has left, or an event is in the past, you would normally use "that":

• "That was a bad year for me".

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In your examples, you should try and match the tense of the previous statement, so:

Dolphins have a smooth, streamlined body. This helps them accelerate up to 34.5 mph.

Because you said Dolphins have a smooth, streamlined body, you are speaking in the present tense, so it is natural to say "this".

If your previous statement had been about something historical, then it would be more appropriate to use "that", for example:

Tyrannosaurus rex had tiny hands. That must have made it difficult to pick things up.

Your second example is speaking about something that occurs in writing, and although it happens only "sometimes", the statement "diminutives supersede official names" is in simple present tense. For that reason, I would also favour "this":

Sometimes, even in formal writing, diminutives supersede official names. Examples of this include the name of the 39th president of the United States Jimmy Carter.

Answer 2

It's true that they're opposites/antonyms, in ONE sense... But, in another sense, the 2 examples clearly constitute situations in which one should use "this/these" (heavier focus) rather than "that/those" (weaker focus/greater distance)

In both examples, the first sentence introduces the topic (the thing focused on), and then the second sentence elaborates on the topic. Compare "this" vs. "that" to "the" vs. "a". Just like you'd say "/I bought A dog. [the existence of the dog is introduced/established). THE (not "that") dog is cute," a native speaker would likely say "Dolphins have a smooth, streamlined body [establishes/introduces their body type]. This [body type]..."

In both cases, the first sentence establishes something as the item to focus on, and the second sentence takes that item as the grammatical and semantic (mental, let's say) subject/focus of the next sentence. Being associated with physical and mental closeness, "the/this" is a better option here than "a/that".

This is because "this" (as opposed to "that") does a better job of preserving coherence. Functional linguists would say that "this" (close/near) is associated with stronger focus than "that" (distal/far) is. Things like "I touched X. This/X was..." sound more "coherent" (more smoothly linked to one one another) than does "I touched X. That was..."