Hello everyone!
"Only when darkness had again descended upon me should I realize how much I had left unseen. " (From Three Days to See)
Could you please tell me about this sentence that
Why is there "had descended" being used with "should I realize" instead of "should I have realized"?

I've learned that when the clause uses "had done" to assume about the past, the main clause is gonna be "should have done" or "would have done". (Is this right fundamentally?)

Then why the sentence have the organization like that?

I would appreciate it very much if you could do me a favor.[Image]

Question

Hello everyone! 
"Only when darkness had again descended upon me should I realize how much I had left unseen. " (From Three Days to See) 
Could you please tell me about this sentence that 
Why is there "had descended" being used with "should I realize"  instead of "should I have realized"?

I've learned that when the clause uses "had done" to assume about the past, the main clause is gonna be "should have done" or "would have done". (Is this right fundamentally?)

Then why the sentence have the organization like that?

I would appreciate it very much if you could do me a favor.[Image]

Answer

This is an example of one of the less common meanings of "should".  "Should" can be used to emphasize how surprising an event is. The meaning of your sentence is the same as
"Only when darkness had again descended upon me DID I realize..."
with the exception that "should" indicates more surprise than "did" does.  With this in mind, all the tenses in the sentence make sense.

Answer

What's more, I feel like it's an assumption about the future for the meaning of it and the context. 
If so, why not use "would descend upon me" and "should I realize"? 
also not" had descended upon me " and "should I have realized"? 
And it use "had descended upon me" and "should I realize"?