The usage in question, exemplified by the statement If you would have told me, then I might have known, is offensive to the educated ear and generally rejected by modern commentators, this one included. The error is often characterized as an improper "sequence of tenses", i.e., a violation of the rules relating the tense of a subordinate clause to that of a main clause. However, as we shall see, tense has nothing to do with it. In fact, the objection seems to have no firm grammatical basis at all, but rather is simply a matter of idiomatic preference. A cursory review of the theory and history of the subjunctive mood, though it lead somewhat afield of the problem at hand, should be illuminating.

In contemporary English, the subjunctive mood, which is variously used to express uncertainty, contingency, obligation, or necessity, is limited to four tenses: present, present perfect, past, and past perfect. Its syntax is further simplified by the absence of any variation with respect to person or number, i.e., there is only one subjunctive form of a verb in each tense. The present tense takes the form of the simple infinitive—be, give, etc.—while the other three coincide with the corresponding plural forms of the indicative mood, e.g., have been, have given (present perfect); were, gave (past); and had been, had given (past perfect). We shall refer to these as the pure subjunctive forms of a verb, as distinguished from formations involving modal auxiliaries, which occur in the same four tenses and perform much the same functions. In particular, the present tense auxiliaries can, may, will, shall, and must are combined with simple and perfect infinitives to form present and present perfect tense verbs, respectively (e.g., can be and can have been); the corresponding past tense auxiliaries—could, might, would, should, and must—are similarly used to form the past and past perfect tenses (e.g., could be and could have been).

The pure subjunctive is gradually being replaced in modern English by these modal auxiliaries, which offer the advantage of greater expressiveness through finer shades of meaning. For example, in the sentence Though he be here today, his plans for tomorrow are in doubt, which reflects standard usage of perhaps a century ago, the subjunctive be would today probably receive modal assistance, depending on the intended meaning: may be to denote possibility or permission, can be to denote ability or capacity, or must be to denote necessity or command. Either of the auxiliaries will and shall could also be used here, but these two have lost much of their power to express modality as they have evolved into signs of the future tense (which has not always been a feature of English—the Saxons, for example, were apparently content to use the present tense to refer to future events). Thus, Though he will be here today, ... would at one time have been taken as an expression of volition or determination in the present tense, and Though he shall ... would similarly have been used to express obligation or likelihood. Today, however, both constructions are more likely to be interpreted as statements in the future tense and indicative mood. *

The semantics of subjunctive tenses can be confusing because they may be used to express either modal remoteness or temporal priority. In the example above, replacing the present tense verb (may be or can be) with the past tense (might be or could be) would reflect a reduction of the likelihood of his being here today rather than a temporal shift. In counterfactual propositions, the past tense generally signifies the present time (If you told me, then I might know), whereas past events are indicated by the past perfect tense (If you had told me, then I might have known).

With regard to our original problem, it is instructive to observe that in older English, the pure subjunctive was used consistently in both antecedents and consequents of counterfactual conditionals. This pattern survives today in poetry and is used occasionally in prose for archaistic effect, as in the following line from Max Pemberton:

It had been no surprise to him if she had fallen dead at his feet. *

It were happy if we studied nature more in natural things, and acted according to nature; whose rules are few, plain and most reasonable. *

And though a linguist should pride himself to have all the tongues that Babel cleft the world into, yet if he have not studied the solid things in them as well as the words and lexicons, he were nothing so much to be esteemed a learned man, as any yeoman or tradesman competently wise in his mother dialect only. *

If it were done when 'tis done, then 'twere well It were done quickly. *

If he were dead, you'd weep for him; if you would not, it were a good sign that I should quickly have a new father. *

In contemporary standard English, the pure subjunctive has completely disappeared from conclusions of counterfactual conditionals. With respect to antecedents, the trend has been slower to take effect, but may be observed with increasing frequency. Thus, the past perfect If you would have told me is still unacceptable, but the simple past If you would be so kind is considered quite proper. Moreover, If you would be and If you could be are often preferred to If you were to be because they are more precise in meaning.

Why, then, is If you would have so obviously incorrect? The literature offers an abundance of unsatisfactory answers. According to Bryan Garner, "Would have for had," as in It would have been more eventful today if we would have won, "is an example of a confused sequence of tenses." But this is the end of his explanation—he is unwilling to characterize the problem any further or to explain how the substitution of the past perfect had won for the past perfect would have won yields a more proper sequence of tenses. Nor does he provide any clue as to why the variant It would have been more eventful today if we could have won, which would appear to be grammatically equivalent to his example, is somehow more acceptable.

Kenneth Wilson is so perplexed by the verb formation in If only we would have known—as if he has never encountered it in other contexts—that he condemns it as a "redundant" use of auxiliaries and facetiously classifies would have known, along with had have known, as an instance of the fictitious "plupluperfect" tense.

Edward Johnson, who agrees with Garner's diagnosis, observes that different tenses are called for in the antecedent and consequent of a counterfactual conditional. He then valiantly defends this bizarre claim in the face of what should be compelling refutation:

"If you would pay me, I would be grateful" is correct, which may seem to contradict the point I have just made. However, the would in the first clause is not just an auxiliary verb, as it is in the second clause—it is the subjunctive of the independent verb will, meaning to wish or to be willing.

It is quite natural to try to explain rules of usage in terms of grammar, but it should be clear by now that there is no grammatical or logical basis for this particular prohibition. Once we have accepted If you would be along with If you could have been, logic would dictate that we learn to live with If you would have been. But idiom supersedes logic, and this construction has simply not found its way into English idiom. There is little doubt that this will eventually change, but in the meantime, there is no compelling reason to embrace the usage in question or to forfeit one's moral superiority over those who do.