Give an example of the correct usage of the pluperfect subjunctive to express regret.

The pluperfect subjunctive is used to express regret about a past action that cannot be changed. It expresses a wish that something had happened differently. A common example is: 'Avrei voluto che tu fossi venuto alla festa' (I wish you had come to the party). In this sentence, 'avrei voluto' (I would have wanted) expresses the desire, and 'fossi venuto' (you had come) is the pluperfect subjunctive of the verb 'venire' (to come). The speaker is expressing regret that the person did not attend the party. The pluperfect subjunctive indicates that the action of coming to the party did not occur in the past, and the speaker now regrets that fact. Another example is: 'Mi sarebbe piaciuto che avessi accettato il lavoro' (I would have liked that you had accepted the job). 'Avessi accettato' is the pluperfect subjunctive of 'accettare' (to accept), indicating regret that the person did not take the job.