What is the difference between the present perfect and the past perfect?

The present perfect is used to express an action in the past that could still be going on until this day. For example: "Ich habe seit seit gestern nicht gegessen". To form it, you need to use the auxillary verbs haben or sein and the past participle. The past perfect is used to express an action in the past that took place before a specific point. For example: "Bevor ich angegriffen wurde, hatte ich keine Angst gehabt". To form it, you need to use the imperfect form of haben or sein and the past participle.