Okay, the truth is: German has no imperfect tense. And neither a future time. And Konjunktiv I and Konjunktiv II aren't really orthogonal to Indikativ. Those categories are a mess. They are much better suited to Latin and Romance languages and make a hell of no sense in Germanic languages. But here we are. Shoot.
If you want to build a schematic with orthogonal axes, it must be one axis non-past (simple tenses) vs past (perfect tenses) and one axis that lumps together mood and tense. This latter axis is about the intent of speech. Like this:
- Präsens / Perfekt — facts
- Präteritum / Plusquamperfekt — narration
- Futur I / Futur II — assumptions
- Konjunktiv I / Konjunktiv I Perfekt — hearsay
- Konjunktiv II / Konjunktiv II Perfekt — counterfacts
- Konjunktiv I Futur I / Konjunktiv I Futur II — hearsay assumptions
- Konjunktiv II Futur I / Konjunktiv II Futur II — replacement for Konjunktiv II / Konjunktiv II Perfekt
Futur II is a perfect tense. Those hearsay assumptions are seldom used and Konjunktiv II Futur I / Konjunktiv II Futur II you have probably learned about as the würden-Konjunktiv II.
On top of that simple scheme, Northerners use Präteritum in place of Perfekt for the auxiliaries, the modals and a few very common verbs. Not more than a handful. The more southern the speaker is from, the less verbs are mangled that way.
And that's all. Does it blow your mind? Very good.
(Okay, that's not really all. Some southern dialects have the concept of double perfect instead of Plusquamperfekt for example because they don't like the Präteritum forms at all. Not even for haben and sein. But that's dialects and you should forget about it for now.)
So if you wonder how to phrase something, first ask yourself if you want to mark the action as completed. As that means you have to use a perfect tense. Otherwise use a simple tense. And then ask yourself whether you want to mark something as facts, narration, an assumption, hearsay, or as a counterfact. Then select the proper mood/tense combination.
Whew … so simple! Why don't they teach it that way?
I have no idea.