Had Romeo known about Juliette's potion, he would not have killed himself.

This is a conditional with had instead of if (You can also say If Romeo had known...), so there is inversion. You put the (first) auxiliary verb before the subject, Had Romeo, and then the rest of the sentence stays the same. The second part is a normal clause where the verbs follow the subject he would not have killed. Not goes after the first auxiliary verb. Had Romeo known about Juliette's potion is the subordinate clause.