Do the conjuntions "only" + "after which" take subjuntive?

No, “only after which” doesn’t require the subjunctive. Your example uses “were,” which is subjunctive here, but that’s because of the hypothetical “Y were real,” not the conjunction itself. So, it’s more about the clause after “only after which.”

This is a long, long way from a grammatical English sentence. Y will be real only after X happens. Or, if you want to raise X in the sentence, perhaps because it's already the topic of conversation: Only after X happens will Y be real. But that latter construction is hard to understand, and should only be used if you're already talking about X.

You need to use the future real conditional, i.e. >X will happen, only after which Y **will be** real. For example: >You will finish your exams in April, only after which your summer vacation will begin.

> X will happen, ***only after which*** Y **were** real. i have no idea what this is supposed to mean.

This construction makes no sense to me. Can you give an actual example of what that would look like? I can not think of any instance where this would sound natural.

That sounds off to me. You wanna use “will” im both clauses, so it’d be “X will happen, only after which Y will happen.” Or, the second clause might just use the present tense (e.g. He’ll give you a signal, only after which you leave the room.”).

Thanks guys. I've looked it up and most the examples were taking the past simple which, your suggestions given, doesn't mean it fully takes the subjuntive. It's funny how adding some grammars would unsettle all the previous knowledge.