Actually, most of the time it doesn't do anything of the kind. Most teachers have settled on a particular way of explaining a particular theory, which the average student accepts as the complete and only truth - at least until they have to apply it, at which point they find it vague, confused, partly true, only sometimes true...or completely wrong.
Or indeed utterly meaningless - nevermind that the examiner gave them full marks for reciting the approved formulations with the standard examples.
But the student doesn't need the theory for anything more than passing the exam, because they use a different, implicit theory in the real world. And the teacher doesn't need deeper understanding because they're not paid to teach anything that deep. And their boss doesn't need a teacher who rocks the boat by disagreeing with the textbooks.
However, my job is not to teach students to pass exams in English. My job is to teach students English. For use in the real world. Which means I've got to understand what the grammar of English really is, not what the books say. Which means...
...I've pretty much got to write my own textbook. Which I do by taking what I learn about grammar at school, and changing it until:
(1) it fits all the real-world examples I can find, and
(2) I can't invent any plausible counter-examples.
Still with me? Well...nevermind. This is what I learned from books with 'Grammar' in the title, about conditionals in English:
A conditional is an "If". It's an utterance containing two sentences, called the antecedant and the consequent, preceeded by the word "If", and separated by a comma (and sometimes the word "then").
There are four kinds of conditional, labeled "Zero", "First", "Second" and, amazingly, "Third".
In a Zero conditional, the antecedent and consequent are both in the Present-Simple form, for example:
If you heat water to 100 degrees, it boils.
If someone eats a pound of arsenic, they die.
If Brenda eats too many cream cakes, she puts on weight.
These express laws of nature, or consequences that always follow from the antecedents.
In a First conditional, the antecedent is in the Present-simple form, and the consequent in the Future-simple, for example:
If you drive too fast, you will crash.
If your child reads this book, they will do better in school.
If the sun comes out tomorrow, we'll go to the beach.
This is used for consequences that are likely, but not certain.
Third conditionals are a Past-simple antecedent, and a consequent composed of the word "would", plus a present simple. Examples:
If he proposed marriage, I would accept.
If she took singing lessons, her singing would be much better.
If we all worked less and loved more, the world would be nicer.
And finally the third conditional, which is a Past-perfect antecedant, and a "would have" followed by a past participle in the consequent. Examples:
If I had known you were coming, I'd have baked cake.
If we'd known, we'd have acted sooner.
If Mary had listened in school, she wouldn't be working now as a waitress.
This form is for things where the antecedent was once possible, but didn't happen, so the consequent never came to pass.
There are also things known as "mixed conditionals", most commonly the 2/3 conditional, which has the antecedent of the Second and the consequent of the third, for example:
If he proposed marriage, I'd have accepted.
These are really just disguised Third conditionals. They're a regional and class variation that you should be aware of, but not use.
Okay, did any of that stir distant memories of grammar classes? And if it did, what's wrong with it?
Well, here's some conditional sentences:
If she starts singing again, I'm leaving the room.
If the film's already started, I won't watch it.
If I saw them yesterday, I've forgotten.
If I saw them tomorrow, I'd say hello.
If she's been dating him, we should meet him soon.
If the world ended tomorrow, I would spend the day eating ice cream.
If the world ended tomorrow, I'd have wasted my time preparing this lesson.
If the world had been ending yesterday, Rupert Murdoch would have been trying to make a profit out of it.
According to the above model, none of these are possible. So as my old political friends use to say, either the theory's wrong, or reality's wrong, and which do you think is more likely?
So what's my alternative? Well, stay tuned, and as soon as I've got the strength to write it up, I'll post it.