"once he had finished" drives me crazy

Question

I'm aware that I might be foolish to ask this rather simple question but it's stunning me ever since I've finished reading the Goblet of Fire.

In Harry Potter and the Goblet of Fire:

Pre-context:

Once or twice, Sirius made a noise as though about to say something, his hand still tight on Harry’s shoulder, but Dumbledore raised his hand to stop him, and Harry was glad of this, because it was easier to keep going now he had started. It was even a relief; he felt almost as though something poisonous were being extracted from him.

The next sentence:

It was costing him every bit of determination he had to keep talking, yet he sensed that once he had finished, he would feel better.

I'm not absolutely sure how to explain to myself why the past perfect here is correct. This is definitely a conditional but a rather strange one to me.

I somehow want to write it as:

• ...yet he sensed that once he would have finished, he would feel better.

or even...

• ...yet he sensed that once he would finish, he would feel better.

To me "once he had finished" sounds as if "he did actually finish it once in the past".

Answer 1

In reported speech, we backshift the tense of verbs.

she said "I am ready"
she said that she was ready.

It is called reported speech because it probably occurs most often with the verb "say*, as in my example, but it is actually used with a whole lot more verbs. There are a lot of verbs in that list: sense isn't among them, but it should be there. In this context, sense means feel, which is in the list.

We use present perfect I have XXXed to talk about something that was started in the past and is not yet completed: once I have XXXed refers to what's going to happen when it is completed in the future. In reported speech, once I have XXXed backshifts to past perfect once she had XXXed.

She said "Once I have painted my bedroom...."
She said that, once she had painted her bedroom....

We use backshift with sense: your sentence uses past perfect. Both simple past and present perfect backshift to past perfect, so we need to decide which was meant:

he sensed that once he had finished, he would feel better. - backshifted
he sensed "once I finished, I will feel better". - simple past
he sensed "once I have finished, I will feel better. - present perfect

It's not difficult to work out that the correct reverse-backshift is present perfect.

Answer 2

Past Perfect is used because the whole narration is in Past Simple: "made a noise", "raised his hand", "was glad", etc. Most importantly, "he sensed that...."

So you can easily "divide" what follows "by Past Simple" (because it's de facto reported speech). What is left after this (if he were sensing it right now) is,

Once I have finished, I will feel better.

Then, if you still have a question of why Present Perfect is used, remember that Present Perfect describes a fact that is completed by a certain time that is relevant. So it is actually a perfectly acceptable and grammatical way of underscoring the sense of "once I have this under my belt", "once this is a done deal", then something will... whatever.

It is actually used quite often. E. g.,

"I generally start off being very uncomfortable with the idea of approaching strangers but once I have talked to a few people, it becomes less of a concern..." [source]

"Once I’ve approached a candidate about a job with one of my clients, and submitted their resume, I will let them know the outcome as soon as I hear back from my client." [source]

"Once you've completed a first draft of your animation, you should run it in presentation mode." [source]

It is pretty clear from all these examples that this is not about something that actually happened in the past. It's about discussing of what happens in a certain situation, under condition X (either usually, or we are discussing some kind of immediate plan). So it's one of the many conditionals :)

P. S. Since it seems to me that the source of your confusion might also be the sense of the word "once" (and you might be thinking of sentences like "I used to be an active child once")—here "once" is used in temporal sense. If means "after".