No matter how many times I've studied this point of grammar, I am still struggling with using it properly.
I was taught that I should use the unrealistic subjunctive (like 'would've written') for something that is only a failed possibility in the past, that is, it never happened, but is merely a product of your imagination projected on the past.
And I was also taught that the realistic subjunctive (like 'would write') should be used for such cases when something has a chance of happening, but may also not happen; therefore, it is also a product of your imagination, except it is applied either to the present situation or to the future, and never to the past.
But what about such cases when we talk about an outcome in the present of some situation that never happened in the past?
For example, here we have a conversation in American family:
Son: - Dad, if the Beatles didn't come to the USA, what would happen?
Dad: - Then, I guess, there would be no British invasion and we would still listen only to rock-and-roll music today.
Son: - Dad, if the Beatles hadn't come to the USA, what would've happened?
Dad: - Then, I guess, there would've been no British invasion and we would've still listened only to rock-and-roll music today.
On one hand, the Beatles did come to America, so the son is talking about a situation that never took place in the past; thus, it's only a product of his imagination. But on the other hand, he is inquiring his Dad about the outcome (of that situation that never happened) as if it were now in the present. In other words, he he extends his imagination to the present.
So what kind of sequence should I use here then? Should I use:
If they hadn't come, then we would've...
If they didn't come, then we would...