Replacing the conditional with the past subjunctive

[Premium]

Originally Posted by aleCcowaN

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-

That's quite useful for me, too. Thank you

[Ivy2937]

También se puede usar el imperfecto de indicativo :

Nunca pensé que esto podía pasar.

[aleCcowaN]

Nunca pensé que esto podía pasar ---> I never though it to be possible -as it is- <and not "... it was possible" nor "... it would be possible">

Though we have to be very careful before stretching the use of the verb poder -argued to be a modal verb in Spanish- to ordinary verbs, we have three examples that can be used in the same context with similar meaning, but showing the specific features of every tense which translate not much as nuances but as hints of what's in the speaker's mind:

... pudiera pasar ---> departing from the basic value of Spanish subjunctive (to be an action killer) it shows two facts mutually exclusive: my construe of a reality that doesn't include such possibilities and reality itself which do allow those to happen. Subjunctive allows both matter and anti-matter to coexist, so to speak, and both "realities" to be true at the same time. In that way we are informed that "it did happen indeed" or "it is possible or customary for it to happen" regardless I wasn't aware about that.

... podría pasar ---> departing from the potential value of conditional, it shows a chain of events starting with the denial of a possibility and later that possibility becoming an actual fact -of me becoming aware of my initial misconception-. In that way we are informed of my state of mind in the past.

... podía pasar ---> departing from the imperfective aspect and its characteristic "fuzziness" regarding the beginning and completion of an action, it shows both action happening -what actually happened or happens and my ignorance or disbelief about it- but it avoids the conflict of two mutually excluding realities -what subjunctive accomplishes- by contrasting the perfective and imperfective aspects: the kind of facts implied by that imperfect -without a clear beginning or end for them and then somewhat unrestricted- is shown against the perfective aspect of past simple, that is, it is shown that my original conception has come to an end.

You can see all three phrases may communicate the same situation provided the context contributes with additional elements. For instance "nunca pensé" instead of "no pensé" or "no pensaba" is a very powerful bit that points to my state of mind and not to my practical knowledge.

[Ivy2937]

As you say this can be tricky but I would give a try to pinpoint the usage of the subjunctive, the imperfect indicative and the conditional for this particular construction
Nunca pensé que esto podría pasar

Main clause verb = simple past and the subordinate clause in conditional tense.
The meaning if you are talking about a past action (afterthought) the conditional suits the idea of the afterthought about a past state of mind about a past action (infinitive pasar with podría a Spanish Modal) . If the speaker thinks about this action after an 'accident' the accidente belongs to the realm of no real at the present moment short or long after it happened.
Pudiera ( imperfect past) is also germane since the subjunctive is the mood of no reality, wishes, opinion, etc).

Podía is an imperfect past the action is not finished into the past and if you refer to a past not finished fits the realm of not truth yet.

[Ivy2937]

<<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'?>>
The imperfect subjunctive (-ra) has connotations of past, present and future) We have to recall that the subjuctive mood does not have time clear-cut limits as the indicative mood. And aslo its origiin (imperfect subjunctive) from the Spanish indicative mood, Pluperfect indicative or
antecopreterite (Había comido).

According to context and timing the answer is yes :
Yo comiera más pero no tengo hambre
Yo comería más pero no tengo hambre

<<<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.
¡Nadie lo creería!

¡Nadie lo cree!
Nadie lo creerá

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

Here temíamos is imperfect indicative the action from the past is not yet finished) it has the sense of durative action therefore non factual yet, the imperfect subjunctive is germane. The conditional usage would give a tint or shade of possibility about something not finished (imperfect indicative or copreterite= temíamos)

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.>>

No es sorpresa ( Present denial) non factual the subjunctive germane)
No es sorpresa que viniera/venga aquí. The speaker is already in the place. The conditional is future from the past. And the main clause is present.
Also would these sentences work just as well and be gramatically correct using the conditional?
No sería sorpresa que llegara Sue
No será sorpesa que llegue sue
Thanks in advance for any insight or suggestions.