English modals are subtle beasts, and nearly all forms can and do occur in different parts of conditionals, but with nuanced meaning. Obligatory crosslink.
If you would
Remember that If you would X always means If you wish/want to X , as in
If you would please take your seats, then we can get started.
Because this is talking about current desires, it is in the present, despite the preterite inflection of would. All uses of would in the “if” part mean that, as far as I’m aware. Sentences like
If you would like to help us, do please give us a call.
are perfectly normal. But this is never a hypothetical. It means
If you do wish to help us, do please give us a call.
But it uses the backshifted would to take the edge off of it and make it seem more polite, less required. It softens the impending imperative that way, since blunt imperatives are always at risk of seeming too pushy.
German speakers regularly get this confused in English and try to use a hypothetical would in the “if” part, which is “not allowed”. Perhaps some Dutch speakers do as well, and this is what your wife was warning you about.
If you could
With could, two possibilities exist, one volitional in the here-and-now, the other hypothetical in the might-yet-be.
The first occurs when If you could X means If you can/are able to X, softened by backshifting can to could. Here it remains a matter of volition, of wishes and wants.
If you could please take your seats, then we can get started.
As you see, this is talking about the present, not about the past or future. And it is not a hypothetical at all, but talking about a want or a wish in the present in the same way that If you would always does.
But with could, that’s not the only possibility. Could can also signal a hypothetical, and the hypotheticals take a past tense form, or the old imperfect subjunctive were in the case of be.
The counterfactual version of could takes a different “then” part:
If she could get off work tomorrow, would she still need a babysitter?
If she were able to get off work tomorrow, would she still need a babysitter?
Notice the would in the “then” part there. This is a paired hypothetical.
If you should
On the other hand, If you should X is a more indirect way of using a simple present tense version.
- If he falls, call a doctor.
- If he happens to fall, call a doctor.
- If he should fall, call a doctor.
- Should he fall, call a doctor.
- If he should happen to fall, call a doctor.
- Should he happen to fall, call a doctor.
- Should he chance to fall, call a doctor.
Notice those are all imperatives in the “then” part. Here’s a literary use, because real examples are always best:
Should you chance to see a knight laugh, or smile, or even, look you, arch his brows, or purse his mouth, or in any way show surprise that I should uphold the Lady Mary, you will take particular note of his name, his coat-armor, and his lodging.
—Arthur Conan Doyle, The White Company
Notice that the “then” part is using the modal will in its deontic imperative mode there, not in its epistemic future mode.
All that notwithstanding, you should please be aware that that citation’s second use of should in the subordinate clause governed by show surprise that is something else altogether, the sort of thing other European languages will often use a special subjunctive inflection for, should one be available in them.*
* And yes, those really are two more completely different shoulds. I did that on purpose.