Replacing the conditional with the past subjunctive
 

Would it be true to say that the conditional tense may also sometimes be replaced by the past subjuntive, and can often be translated as 'would'?

Or is there a better way to put it? or some specific grammar rule governing this use?

I'm talking about examples like ...

¡Nadie lo creyera!
Nobody would believe it.

Temíamos todos que se precipitara al río.
We all feared that he would fall into the river.

and this one where the governing verb isn't in a past tense

No es sorpresa que Breeanna viniera aquí.
It's no surprise that Breeanna would come here.

Also would these sentences work just as well and be gramatically correct using the conditional?

Thanks in advance for any insight or suggestions.

The answer is 'yes' but I'll let a native speaker say something because I might get it wrong.

I just thought I would comment about terminology. The conditional and subjunctive are not tenses, they are moods. A finite verb has 5 attributes: number (sg.,pl); person (1,2,3); voice (active, passive); tense (present, future, etc.); and mood (indicative, subjunctive, imperative, interrogative and conditional). You describe a verb exactly by specifying a combination of these attributes. :)

It's a matter of case by case.

"¡Nadie lo creyera!" is unusual in actual language in present or future situation, whether it works as a conditional or not. Talking about past contexts it can be used with a basic meaning of "nobody believed it!" where subjunctive mood reinforces the "didn't happened" component.

Anyway, actual speakers seldom choose these ways unless there's a need in the speech to defocus that part, as in:

Les dijo que le habían asaltado unos gitanos, y como nadie le creyera, cambió su versión por una que los sustituía por unos cazadores furtivos vestidos de manera inusual.

Temíamos todos que se precipitara al río.
(Wouldn't it be "We all feared that he might/could/would fall in the river", with might or could for an involuntary fall, and would for an intentional action? Spanish doesn't distinguish by merely using subjunctive if context doesn't help)

No es sorpresa que Breeanna viniera aquí
(It's no surprise that Breeana came here -?-. It clearly refers to an action in the past: Breeanna came indeed)

Thanks guys,

@ Perikles - I totally agree about the subjunctive being a mood and I have tried to research whether the conditional is a tense or a mood so your explanation about verbs was great. From what I found I was under the impression that even grammarians can't agree on whether the conditional is a tense or a mood, for example I read this article leaning towards it being classed as a tense:

The Spanish conditional — although semantically it expresses the dependency of one action or proposition upon another — is generally considered a "tense" of the indicative mood, because, syntactically, it can appear in an independent clause.

@ Alex

Thank you, your examples were very helpful. I'm still trying to get my head around how falling in the river could be intentional jeje but I get the rest :)

My mistake, precipitarse means either fall (from a high point, like a cliff or a zeppelin) , throw oneself, or rush (hurry) and the use is semi-formal for throwing oneself and almost-formal for falling from the heights. Precipitarse al vacío is a common fixed expression describing what happens when a person throws him or herself from a window on the 40th floor or an climber loses his grip to a steep rock face and all the safety equipment fails.

That is nonsense to me, because you don't need an indicative to make an independent clause. Ultimately though, the labelling is a matter of choice.

What I got from the quote was that an independent clause would need to have a verb in a certain tense and given that the conditional can appear in an independent clause it would be a tense of the indicative mood.

But as you say it's just a label.

So this model sentence from the daily calendar page, Nunca pensé que esto podría pasar. really should be ...pudiera pasar., right?

I think you're right in this case. But it's confusing because there are other seemingly similar cases in which you use conditional--for example, you would say "Nunca le dije que podría hacerlo." The idea of thinking of indicative versus subjunctive as a mood helps. Indicative mood is something you think has an element of objectivity, truth, or concreteness, so "I never said I would do that"--I never told you something would happen in the future. But "Nunca pensé que esto pudiera pasar" is "I never thought this would happen," indicating doubt (at the time of the first verb) so it's subjunctive. At least--I think so; someone else might be able to elaborate.

Better late than never :)

Nunca pensé que esto pudiera pasar ---> I never thought it could happen -as it did, indeed-

Nunca pensé que esto podría pasar ---> I never thought it could happen -it did happen or maybe it didn't-