I have seen this tense used in principal clauses many times, but I am not certain of its precise meaning. I saw it frequently in Albert Camus. Here is a passage from Le Petit Prince: "Cette histoire de griffes, qui m'avait tellement agaçé, eût dû m'attendrir..." Could the author also have written, "...aurait dû m'attendrir...?" If so, what is the difference in meaning. I would be grateful for any explanation, especially if one could direct me to a grammar site that deals with this construction. Many thanks.