What is all this strange talk about past tense to indicate future? You are dealing with the conditional here, a hyothetical situation introduced by the word 'imagine': what you imagine has not happened,
Well, we tend to use the word 'conditional' of utterances containing such expressions as 'if', 'unless', 'providing', etc. The utterance we are discussing is certainly hypothetical; it is not conditional.
"Imagine you would have to go to another country and learn German." There is nothing wrong with this sentence.
There is nothing catstrophically wrong with it. It's just that 'Imagine you had to' is correct. If Danman were taking a Cambridge exam, he might lose a point if he used the first construction.
'had to' = 'müsstest' 'would' = 'würdest'. That is the difference.
It matters not a jot what happens in another language. We are talking about what happens in English.
Stell dir vor, du müsstest in ein anderes Land ziehen und Deutsch lernen. Just by looking at the verb form in the translation, you can easily see that you are dealing with the Konjunktiv II.
The English may well be rendered by the 'Konjunktiv II' in German. That is irrelevant.
There are no past tense verbs in your sentence.
:-(' You are simply wrong. 'had' is a past tense form. You might wish to call it a 'past subjunctive', but it's still a past tense form.
ẃould' is the past subjunctive of 'will'.
So, even by your analysis, it's a past tense form:roll:. Some modal constructions may well be rendered by subjunctive forms in other languages; it's not particularly helpful to think of them as subjunctive in English.