How can this be "would"?

Question

I was studying the use of "used to" and "would" when I came across this sentence, which has been nagging me:

1a Family values used to be quite different in those days.

1b Family values would be quite different in those days.

2a It used to be quite normal for cousins to marry.

2b It would be quite normal for cousins to marry.

3a Generally speaking, these marriages used to succeed as well as any others.

3b Generally speaking, these marriages would succeed as well as any others.

Which "b" sentences are grammatically wrong?

According to what the book* said, "used to" - not "would" - can only go along with the so-called past state. With that being said, I was easily able to make out that

1b is wrong, obviously (if we're not dealing with deduction here);

3b is, of course, right, for it's telling a repetition.

But, moreover, 2b is not wrong, which is kind of weird, although I do feel it right somehow. Can somebody explain this to me?

Thank you!

*Grammar and Vocabulary for Cambridge Advanced and Proficiency by Richard Side and Guy Wellman"

Answer 1

Both of these read perfectly well to me.

2b It would be quite normal for cousins to marry is describing a a future hypothetical action (the cousins get married) from a point in the past where the cousins are unmarried.

3b Generally speaking, these marriages would succeed as well as any others is describe a future possibility (successful marriage) from a point in the past when a couple is newly married; it is also in the form of typical/repetitive action in the past.

Answer 2

To clarify things, let's say the sentence was:

Many years ago, it would be quite normal for cousins to marry.

This is perfectly valid. "Would" can be "used to talk about things in the past that happened often or always." In this context, it isn't referring to a hypothetical or future action, but to a habitual action in the past.

Answer 3

"Would" can be used for something that had many opportunities to happen, and did in fact happen in many of those cases. So for instance, you can say "I used to be a cop", but not "I would be a cop", because you being a cop or not is something that happened only once. There are some cases where "would" is even mandatory over "used to". For instance, if you say "Back then, if it rained, I would take an umbrella", you can't substitute "used to" and get "Back then, if it rained, I used to take an umbrella"; you'd have to reword it, e.g. "Back then, I used to take an umbrella every time it rained".

1b is a bit unidiomatic because presumably either family values were different or they weren't. For "would" to work, you would have to be talking about a bunch of opportunities for family values to be different. 2b works because there were many opportunities for cousin marriages to be considered normal (every cousin marriage was such an opportunity, and arguably every marriage at all was an opportunity, as the marriage could have been a cousin marriage). 3b works because each cousin marriage was an opportunity for a cousin marriage to work out.