I'll try and do my best to answer this question, in such a way that you'll be able to make some sense out of this, and then apply these guidelines in the future.
The first sentence is an example of the so-called second conditional
If you were a king, what'd your wife be called?
Rightly so, edyta—the OP—said this is an impossible situation. The chances of edyta, whom I presume is a woman, becoming king is even more remote. But the sentence doesn't refer to a
past event. Being a king never happened and it never will. The idiomatic phrase If + you + were is expressing an unreal situation, similar to saying: Let's pretend you are a king.
The construction If + subject + past and would + infinitive is often referred to as irrealis, which means that the situation proposed is ‘unreal’ or ‘imaginary’
in the present
See also Present Unreal Conditional
The past verb-form is used to suggest that the situation is probable, or imaginary, or set in a hypothetical future.
Martin Parrott provides this insight:
We use them [type 2 conditional sentences] to refer to or speculate about something that is (or that we perceive to be) impossible or ‘contrary to fact’. This is sometimes presented to learners as ‘very unlikely’. The real point, however, is that at the moment of speaking we see the action or event as being impossible.
Source: Grammar for English Language Teachers
Probable; e.g. “If I were on holiday, I'd go to the beach every day.”
Imaginary; e.g. “If I were a cat, I'd catch mice all day long.”
Or hypothetical future event; e.g. “If I had a car, we would drive down to Mexico.”
The OP's example falls under the category of imaginary, If I were a King. . .
The following sentence is an example of a hypothetical future event
A: What would you do if you needed a haircut?
i. I'd go to the hairdresser's.
ii. I'd cut it myself.
iii. I don't know. Cut it, I guess. Why are you asking?
The so-called ‘third conditional’ is for talking about a hypothetical/imaginary situation in the past (which didn't happen) and its consequence.
If you had worked harder last year, you would have passed your exam
if + past perfect and would have + past participle
See also: Past Unreal Conditional
Thus the sentence (note my correction on the word order)
If you had been king, what would your wife have been called?
Expresses either an imaginary situation
in the past that didn't happen, or an event that didn't happen
in the past. In both cases, the situation is ‘impossible’ because we cannot change the past.