@ Brus - "I used to be prime minister, you know".

Does this mean "I" can't remember I was prime minister? No, it just means that I am not the prime minister, but I used to be.

@ Brus - Well, Sundy, it may not be usual for someone to develop dementia but it happens.
Besides all that, the point under discussion is a linguistic one.

Putting the context in the language would make more sense than to discuss purely linguistic instances. I am emphasizing "purely" here.

Though I would have to agree that your interpretation works in your proposed context, but that's not an usual context. Why did some expressions in a language die out? One of the reasons is that there is a lack of contexts in life to fit in.

Trust me, "if I was you" will become more common as it means the same thing as "if I were you" since by no way, no mean, the assumption of "I am you" would become true.

@ Brus -On another linguistic point, what does this mean: "You are kind of trust your girl friend on that she wouldn’t do this ... "? You wrote it. No need to call the police, but it scores poor marks for English, I feel.

You could give me poor marks of English, but not poor marks of language and how a language works.

I wanted to edit my post after submission, but I couldn't. I may have to change it to "you kind of trust your girl friend that she wouldn’t do this ..."

@Warsaw Will - of course you're right, which is why, in EFL, we refer to this as the Unreal past. We only have to compare it with any other verb - 'If he acted like that at my party, I'd throw him out' - this can only be about an unreal event in the present / future - the 'would' in the result clause tells us that. If it was about the past, we'd use a past or present tense in the result clause, as in the cad example (which seems to be very popular in the States).

The context is very important in understanding the subjunctive sentences.

'If he acted like that at my party, I'd throw him out' - this can only be about an unreal event in the present / future - the 'would' in the result clause tells us that. If it was about the past, we'd use a past or present tense in the result clause, …

This may be not true depending on various context. First, this can be about the past as in this context:

You held a big party at your big home with dozens of people attending, where you were very busy all over the place, even without paying attention to your girl friend. Now the next day after the party, one of your best friends tells you that he saw your another friend was trying to impress your girl friend by chatting with her in a small room on the second floor. You are kind of trust your girl friend on that she wouldn’t do this which is seen as inappropriate at your party. But this is what your best friend is telling you, so you might think that your best friend was just mistaking another girl for your girl friend. You might say to your friend:

You might have mistaken another girl for my girl friend, but 'If he acted like that at my party, I'd throw him out' – “I’d “means the future here while 'If he acted like that at my party’ refers to the past, so the whole sense implies that if “he acted like that at my party” is to be proven, when I see him I would throw him out. Please note that “out” here has changed not to mean “my party”, instead, the surrounding where you would meet him when you throw him.

So we have to look at the context (scenario) in interpreting subjunctive sentences. Without doing this it would let the argument keep going and going forever.

@ Brus - No it isn't. It is easy and simple: I am informed (by anyone at all) that this national misfortune has occurred, and I as the speaker am declaring that it should not have been allowed (if indeed it was - the indicative mood of the verb "was" rather than "were" means it is treated by me as an open condition, which is to say that I accept it is possible that I was prime minister at one time, but I can't remember). If I wished to indicate disbelief in such a preposterous assertion I would use a closed conditional clause, denoted by the subjunctive form of the verb: "were".

I would be very nervous if you say that “I accept it is possible that I was prime minister at one time, but I can't remember”. Or if I am serious enough, I may call the police to take you to hospital for an overall medical checkup. You can’t forget if you were once the prime minister some time ago unless there is a medical problem. So this is not an usual context.

@goofy
If I was the Prime Minister, I would change the law.
If I were the Prime Minister, I would change the law.
Don't both of these sentences refer to unreal present events? If I can't remember if I was Prime Minister, I would be talking about a past event. I might say:
If I had been the Prime Minister, I would have changed the law.
or
I can't remember if I was the Prime Minister.

It’s common sense that only the current Prime Minister would have the power to change the law, so the clause “if I was the Prime Minister” refers to the present, since there is no logical connection between being the former Prime Minister and changing the law if you are saying that you were Prime Minister before. But who is the Prime Minister is a fact, in this context, not the speaker, which is true without any subjective judgment, making “if I was the Prime Minister” is a counterfactual supposition. So in this context, “if I was the Prime Minister” and “if I were the Prime Minister” mean basically the same thing.

@Brus
If I was the Prime Minister, I would change the law." This to me suggests that I am surprised and doubtful to hear that I was sometime in the past the Prime Minister, find it hard perhaps to believe that such an thing could have been allowed, and if it is true, would want someone to change the law, maybe to prevent such a calamity in the future.
The logic doesn’t stand. You are linking the assumption of being the Prime Minister in the past to the change of law which would be made to happen in the future. Who would the speaker be talking to? It’s hard and complicated to find such a context that would fit in here.

(I have, of course, by the way, no particular prime minister in recent history in mind.)
"If I were the Prime Minister, ..." has a totally different meaning, that I want the law changed and if I could I would, but I can't so I shan't because I am in fact not the Prime Minister.
Which one do you mean?
@goofy
Your sentence "If I was a hopeless cad, I apologize" refers to a past event, so it's not the same context as my examples.
Would you ever use "if I was" in exactly the same context as "if I were"? Would you say this:
"If I was a hopeless cad, I would apologize."
"If I was a hopeless cad” could refer to a past event too, even in "If I was a hopeless cad, I would apologize". The reason behind is that whether or not you are a hopeless cad depends on the criteria of judgment of the person you are speaking to, involving subjective factor. This is not falling in the same category as being the Prime Minister.