English Conditionals and "would"

I'm having a discussion with my wife on English conditionals.

She says we cannot have "would" in a hypothetical if statement:

If I would want to change my address, should I let you know?

She says that "would" can only occur in the second part (as in Type 2).

If this is the case, is there then no way to describe "a hypothetical situation in which I would want to change my address" as conditional?

I feel like this sentence is different from:

If I want to change my address, should I let you know?

Then again, maybe it is just that I'm confused with how we would say it in Dutch.

And what about if it is the change, not the wanting to change, that is hypothetical? In other words:

If I would change my address, should I let you know?

is there then no way to describe "a hypothetical situation in which I would want to change my address" as conditional?

Yes, there is a way. To express a hypothetical present time situation, use the past tense verb form in the if-clause:

If I wanted to change my address, should I let you know?

If the change is hypothetical:

If I changed my address, should I left you know?

For a hypthetical past time situation, use the past perfect in the if-clause and use the perfect in the other clause:

If I had wanted to change my address, should I have let you know?
If I had changed my address, should I have let you know?

The use of would in the if-clause ranges from colloquial to "acceptable" (i.e., "standard) in American English. The answer by StoneyB seems to cover the uses generally recognized as standard. However, I'm not sure if it includes the situation where the action in the in-clause comes after the action in the other clause, as in

If it would make her dance I would give her a dollar.

Here, the dancing comes after the dollar-giving.

In addition, Longman Exams Dictionary, grammar guide is said to include the following example:

The blockades wouldn't happen if the police would be firmer with the strikers.

Here is conditional would in the if-clause of a typical conditional sentence. I don't have a copy of this book, but it is cited in the Wikipedia article on the English conditional and elsewhere (although it's possible that Wikipedia is dependent on the WordReference link as its source). The same WordReference link also cites Practical English Usage (3rd edition) as saying

Conditional would is sometimes used in both clauses of an if-sentences. This is very informal, and is not usually written. It is common in spoken American English:

It would be good if we'd get some rain.
How would be feel if this would happen to our family?

Thus, it really depends on the register (formal vs informal) that you are asking about to answer your question in general. For colloquial American English, at least, the use of conditional would in if-clauses is used by some and apparently accepted in one sentence by Longman in written English.

However, I don't know of any American who'd say your example sentence (If I would want to change my address, should I let you know?).

Will and would are rare in the protases (if clauses) of conditional constructions. They are acceptable in only a few circumstances:

• In actualization conditionals when will/would has a volitive sense, equivalent to "be willing":

  If you will get the pizza I'll pick up the beer.
  If you would get the pizza I could pick up beer on my way over.

• In actualization conditionals when will has a habitual sense—it's emphatic and implies obstinacy:

  If you will ask smart-ass questions you must expect smart-ass answers.

• In inference conditionals when will/would designates a future contingency accepted (even if perhaps only provisionally) as true:

  If, as you say, the auditors will be here tomorrow, we'd better get to work cooking the books. John declared that if the auditors would be there the next day they'd better get to work on the books.

I have always suspected that the reason for excluding will/would from the protases of actualization conditionals is that the condition must be parsable as prior to the consequence. This doesn't apply in inference conditionals since we can draw inferences about prior events from subsequent ones.

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.}