I Could If I Would

I've noticed an interesting trend in Spanish - at least Chilean Spanish.  Although my suspicion is that this applies in other countries as well.  And I'd certainly be interested in how that manifests.  But I digress...

Background
If you have taken high school or college level Spanish, then you have probably experienced what most Spanish students consider the bane of their existence (or at least this one does!), otherwise known as the "subjunctive."  The reason that I find this particular mood (no, it is not a tense, it is a mood) so challenging is because there is essentially no equivalent in English, which makes it difficult for me to remember when exactly I'm supposed to use it and when I can just fall back on the default indicative (non-subjunctive) mood.

There is, however, a lovely exception to this rule: the imperfect (past) subjunctive, whose use in English is virtually identical to the Spanish usage.

In both English and Spanish, the past subjunctive is used to express a hypothetical situation or wish.  For example, in the phrase, "If I had a million dollars," the word had is in the past subjunctive form of have. (In English, the past subjunctive looks exactly like the past indicative, with one exception.)  In Spanish this phrase would translate to, "Si yo tuviera un millón de dólares"; tuviera is the past subjunctive of the verb tener (to have).

And in both English and Spanish, the past subjunctive tense generally accompanies the conditional tense (sometimes called present unreal conditional). In English, the conditional is signified by the word would, and in Spanish by the suffix -ía.  Take the following example:

"If I had a million dollars, I would buy you a house."
"Si yo tuviera un millón de dólares, te compraría una casa."

In both cases, we have the pattern past subjunctive + conditional, i.e., a hypothetical situation followed by a hypothetical result, outcome, or intent.  Nice and neat!

Of course, you can reverse the order, as is done in the ubiquitous English-language idiom "I would if I could" (often followed by "but I can't so I won't!"), making the order conditional + past subjunctive.  This is perfectly acceptable in both English and Spanish as it does not change the meaning of the phrase or sentence in either language, as long as the "if" stays with the past subjunctive.

However...

Trend
What I have noticed is that Santianguinos have a tendency to use the past subjunctive as if it were the conditional, often resulting in double past subjunctive!  Madness!!

For example, instead of "Lo habría sabido si me lo hubieras dicho" ("I would have known if you had told me"), you'll hear "Lo hubiera sabido si me lo hubieras dicho," which is like saying "I had known if you had told me," which makes no sense!  Essentially, that is like saying in English, "I could if I could."

But that's not all!

No sooner had I started noticing this trend among my Spanish-speaking friends than I also started to notice the same phenomenon among English speakers -- only backwards!!  An English speaker will say, for example, "If I would have known that, I would have gone," when the correct phrase would actually be, "If I had known that, I would have gone."  Apparently, English speakers opt for the double conditional!  Although most native English speakers wouldn't flinch at such an error (except perhaps fellow grammar-police types like myself), it truly is as nonsensical as saying "I would if I would."

To me, these trends show that people unconsciously associate these two tenses to a point that they seem virtually interchangeable.  The interesting part is that this interchangeability only seems to go one way, and which way that is depends on whether you are speaking English or Spanish!

Since these tenses so often go together it does make sense to me that they might get mixed up or one might replace another from time to time, especially since they are not quite so easy to differentiate as, say, simple present and simple future for an English speaker, or present subjunctive and simple future for a Spanish speaker.  For example:

Correct English sentence:
When I finish eating, I will be full.

What you will never hear from a native speaker:
When I will finish eating, I will be full.
When I finish eating, I am full.

Correct Spanish sentence:
Cuando termine de comer, estaré satisfecha.

What you will never hear from a native speaker:
Cuando terminaré de comer, estaré satisfecha.
Cuando termine de comer, esté satisfecha.

(Notably, these are common mistakes for language learners.)

So my question is, what is the difference?  Why does one tense get exclusively substituted for the other in one language, and the complete opposite happens in another?

A final twist in this story that may actually shed some light on my own question is the issue of politeness.  I can think of examples in each language where the chosen "preferred" tense (i.e. past subjunctive in Spanish and conditional in English) is used to express politeness in the respective language.

For example, in English when we would say, "I would like..." (conditional), in Spanish you could say, "Quisiera..." the past subjunctive form of the verb querer (to want).  The conditional form in English and the past subjunctive form in Spanish are both considered more polite than the simple present, "I want" / "Quiero."  (Although it would also be appropriate to say in Spanish "Me gustaría" -- the literal translation of "I would like" -- in some circumstances, the meaning of the polite phrase "I would like" is better expressed in Spanish by the past subjunctive "Quisiera."  You would not say "I wanted" or "I would want" in English, nor "Querría" [literally "I would want"] in Spanish, to express politeness.)

Another example is the phrase "If you would just calm down..."  It is more grammatically appropriate to say "If you just calmed down..." i.e., use the past subjunctive.  But in this case, the grammatically appropriate phrase is socially inappropriate since it sounds much less polite.  The latter sounds accusatory, whereas the former is softened by the conditional would.

Therefore, there appears to be a link between overuse of the preferred tense (either past subjunctive or conditional) and the use of that tense to express politeness.  But is that really the case?  And if there is a link, can it serve as an explanation?  Is there a logical reason why those two phenomena should be related?  Or is it really just a coincidence?

What do you think?  Do you agree with my analysis?  Do you have other ideas?  Do similar occurrences take place in any other languages you know?