Illud

[Bitmap]

My question was on an indirect statement where a subjunctive (passive) verb would be needed as the “not-main” verb.

e.g. “the romans thought the city might be besieged one day.”
Romani creverunt civitatem olim obsessum sit/ esset.

An indirect statement stands in the AcI. Only clauses subordinate to (and dependent on) the AcI are in the subjunctive.
For "the Romans thought the city might be besieged one day", you can write
Romani creverunt civitatem olim obsideri posse.

obsessum sit/ or esset came from “indirect questions”, in my textbook, where a future tense is needed.

I'm pretty sure your textbook would have used a future participle there.

obsessum sit/ or esset came from “indirect questions”, in my textbook, where a future tense is needed. “add the subjunctive of sum to the future participle”.
I thought the same would apply here?

There is no indirect question here.

That would be something like
"The Romans were wondering when the enemies would besiege the city."
Romani mirabantur quando hostes (nom.) civitatem obsessuri essent.

 

[john abshire]

An indirect statement stands in the AcI. Only clauses subordinate to (and dependent on) the AcI are in the subjunctive.
For "the Romans thought the city might be besieged one day", you can write
Romani creverunt civitatem olim obsideri posse.
I'm pretty sure your textbook would have used a future participle there.
There is no indirect question here.
That would be something like
"The Romans were wondering when the enemies would besiege the city."
Romani mirabantur quando hostes (nom.) civitatem obsessuri essent.

I need to clarify.
My textbook used the future participle plus the subjunctive of sum in indirect questions needing a future passive (subjunctive).
So I thought the same construction should work whenever a future passive subjunctive is needed.
How the language works is a primary interest.
But you did answer my question, “will the same construction work for both?”
The answer is no.