Greetings one and all.

I'm quite perplexed about the usage of this/that/these/those when used in formal/academic writing. Not knowing a lot about grammar, I always thought that "this" and "these" were present tenses while "that" and "those" were past tenses. At uni I always try to write in a formal style, so I often use that and those. However, after reading a little on grammar, it would seem as if I was wrong. While I understand that what determines their use is distance, I get confused when I read novels and academic works, where this/that/these/those seem to be interchangable. For example, I often read the terms "these ideas", "those ideas", and I don't understand.

Any help would be welcome!

This/That/These/Those are not verbs and therefore do not have tenses.

They are pronouns.

This is a word : These are words = the first is singular, the second is plural.
"This/These" refer to things that are near OR identify something specific.

This book is very easy to read. These books are heavy to carry.
This cat is a Siamese cat. These dogs are shepherd dogs.
The glasses go on this table. The plates go on these tables.

That was a bird; Those were birds = the first is singular, the second is plural.
"That/Those" refer to things that are distant OR identify a specific person or thing observed or heard by the speaker OR refer to a specific thing previously mentioned or known OR are used in singling out someone or something with a particular feature

The knives go on that table by the window. Those tables by the door do not need them
That man is my sister's brother-in-law. Those men are not related to her.
You know that fact. You have learned those facts
The road you want is the one that has a traffic light on the corner.

Have a look at this: http://esl.about.com/library/beginner/blthis.htm
Teachers: We supply a list of EFL job vacancies
These terms (this/that/these/those) are a little bit interchangable. (For example, I could have said, "Those terms" instead of "These terms.") This and That are used with singular nouns (This idea) and These and Those are plural (These ideas). It has nothing to do with present or past tense. It has to do with distance. One should use This/These rather than That/Those to refer to the topic(s) just mentioned. But it is acceptable to use "Those ideas," for example to refer to ideas one is summarizing.
In academic writing authors often use This without specifying what they are referring to. For example: "We know that learning requires effort. This will help you succeed." What does This refer to? The knowledge that learning requires effort? The effort itself? The goal to learn something? Be clear about what This means by avoiding This or at least putting a noun right after This: "This knowledge will help you succeed."

These friends are coming

"He walked on the street's curb, holding out his arms for balance as though in a high-wire act. This would be a night that changed everything."

My question is regarding "This would be..." whether "This" refers to the present or past by virtue of relationship to the third person voice. Should it be avoided and substituted with "It would be"?

Students: We have free audio pronunciation exercises.

This would be a night that changed everything.

would here is used by the writer to forecast what will happen in the future. It's like the writer has special knowledge of the future. Because of 'would', we understand that 'this' refers to the coming night.

It would be is not wrong.