If you would have told me you needed a ride, I would have left earlier.
I have very vague awareness of the fact that there is a special type of conditional clauses using this pattern: If + would have + past participle. Could you please explain to me how my exemplary sentence differs from If you had told me you needed a ride, I would have left earlier.
The technically correct way to say it is:
If you had told me you needed a ride, I would have left earlier.
As you noted in your question. I would say that formal usages still stick to this and never use "if you would have told ...".
However, there is a way in which there can be "would have" in the conditional:
I you would have just one piece of cake, I would be delighted. (Where the cake is the one I baked, of course.)
As someone else already pointed out, this is not a valid conditional form.
There are three conditional forms, each one with its own functions and nuances:
First conditional: If + present => future simple
This form is expressing a probable outcome in the future. For example:
"If you study, you will pass the exam."
The implication is that probably you will study, therefore probably you will pass the exam when the time comes.
Second conditional: If + past simple => would + verb
This form is expressing an improbable outcome in the future. For example:
"If you studied, you would pass."
The implication here is that you are not studying, therefore you will probably fail the exam when the time comes.
Third conditional: If + past perfect => would have + past participle
This form is expressing an impossible outcome in the past. For example:
"If you had studied, you would have passed."
The implication here is that the exam has already been, you have failed it, and the reason is in the if-clause.
The way to invoke the if clause with would is by using if only.
We use if only + would + base form when we want the behaviour of someone or something to change:
If only you wouldn't speak so slowly.
If only it would snow.
As you were told, the conditionals are formed by differrent structures. Even though there are more forms called mixed conditionals which are a mixture of the second and third type:
If you had had lunch earlier, you wouldn't be hungry now. (I didn't have lunch earlier, so I'm hungry now.)
There are many ways to create mixed conditional forms.
If you would have told me you needed a ride, I would have left earlier.
This sentence is incorrect, because for conditional you never use if + would in the same part.
If you had told me you needed a ride, I would have left earlier.
We use the third conditional, to talk about things in the past that didn't happen but we wish, it happend.
