I had hoped we would leave tomorrow, but it won't be possible

[Mori]

As far as I know, the past perfect is used to talk about something that happened before something else in the past, but in the following sentence, there's only one action in the past:

I had hoped we would leave tomorrow, but it won't be possible.

Source: Michael Swan, Practical English Usage fourth edition, entry 54.2

This is how I justify it: The past perfect helps to convey that the hope (the act of hoping) occurred before the realization that leaving tomorrow won't be possible. I wonder what you think.

 

[Tarheel]

I would use "could" there, thus:

I had hoped we could leave tomorrow, but it won't be possible.

Yes.

 

[teechar]

I had hoped we would leave tomorrow, but it won't be possible.
This is how I justify it: The past perfect helps to convey that the hope (the act of hoping) occurred before the realization that leaving tomorrow won't be possible. I wonder what you think.

That's basically right. The sequence is
hope ---> (some event that disrupted the plan) and realization ---> time of speaking