Moving the verb back a step inside an if-sentence does not move the event back in time. It moves it further from what the speaker is treating as real, which is why if I had a car is a sentence about this afternoon and says nothing whatever about last year.
That single decision is what the numbering hides. A table that runs from zero to three looks like four structures to be learned in order. There are two settings, and the first of them is a slider: how far is the speaker putting this from what is actually the case.
The four, and the four that get confused
- If you heat water, it boils. The zero conditional, for things always true, present tense in both halves.
- If it rains, I will stay. A real possibility. Present after if and will in the other half, and the commonest error is putting will after if.
- If it rained, I would stay. Unreal or unlikely now. The past tense here says nothing about the past; it marks distance from reality.
- If it had rained, I would have stayed. Unreal in the past, and it did not happen. This is the one used for regret and for blame.
- If I had studied, I would be a doctor now. A mixed conditional: past cause, present result, and it is extremely common in real speech.
- unless means if not, so unless it rains is if it does not rain, and adding a second negative reverses it by accident.
- If I were you. The subjunctive survives in English in this phrase and a few others, and was is now widely accepted except here.
- I wish I had more time. Wish behaves like the unreal conditional and takes the past tense for a present situation, which catches nearly every learner.
The past form is the marker for unreal, and it is used elsewhere
Once you see the step back as distance rather than as time, it turns up all over the language and stops looking like a special rule for if. I wish I had a car is the same step. So is it is time you left, said to somebody who has not left yet. So is the English of a polite request: I was wondering whether you could is not a report of what you were doing a moment ago, it is the same past form being used to stand back from the asking. A learner who has been taught this only as conditional grammar meets those three and files each one as an exception.
The second setting is separate and it is where the numbering does real damage. The if-clause sets the distance. The result clause answers a different question, which is when the consequence lands, and nothing forces the two to match.
Where it stops: the first mixed sentence anybody actually says
Learners hold the table for about a year and then meet if I had studied medicine, I would be a doctor now. The if-clause is shaped like the third pattern, the result clause like the second, and the sentence is not on the table at all. The usual conclusion is that English is inconsistent here, and the usual response is to avoid the form, which is why plenty of confident speakers at a high level never produce a mixed one in their lives.
Nothing is inconsistent. The studying did not happen, which is as far from real as the language goes, so the if-clause takes two steps back. Being a doctor is about today, so the result clause stays in the present. Set the two halves independently and the sentence builds itself. Set them from a table of four and it cannot be built at all.
Hearing the one that is nearly inaudible
The far end of the slider is the hardest to catch in speech, because had contracts onto the pronoun and almost disappears: if I'd known takes about as long to say as if I know. YouGlish is the cheapest fix. It searches spoken video for a phrase and plays the clips back to back, free and with no account, and it replaced hunting for one usable example in a coursebook recording made by two actors in a studio. Search if I'd known, listen to twenty speakers in a row, and the contraction stops being a gap in the sound and becomes a thing you are listening for.
Its limit is worth knowing before you rely on it. It searches captions, so a phrase that captioners routinely mistype comes back thin, and it will show you what people say rather than what an examiner will accept, which are not the same set.
What it looks like when it goes wrong
One error dominates and it is worth naming on its own: if I would have known. It is extremely common in speech, including among native speakers, and it is marked wrong everywhere it is written down. The if-clause carries the distance; the result clause carries would. Putting would in both halves doubles the marking and leaves the sentence saying the same thing twice.
The earliest error, and the one that clears up soonest, is a prediction word inside a real if-clause: if I will see him, I will tell him. Nothing is wrong with the logic of it. The present form already carries the future inside an if-clause, so English refuses the second marker.
The step back you have just met is not only a conditional device. It is the same movement that happens to a verb when you repeat what somebody else said, which is moving what someone said back one step in time, and it lands on the same small set of words that carry doubt and obligation, can, must, should: the verbs that add attitude rather than action. Both sit inside the rules that decide what a sentence is allowed to mean.
The question this leaves is what the result clause is doing, because would, could and might are choosing between degrees of certainty rather than reporting a fact, and that choice is made with the same small set of words in sentences that have no if in them at all.