Learn German: Anna und Peter haben das Projekt diskutiert. - Anna and Peter discussed the project.

This is because the infinitive (dictionary form) of the verb is diskutieren. Verbs that end in -ieren are a special group in German. When you form their past participle, you never add ge- to the beginning. You simply take the stem (diskutier) and add a -t at the end to make diskutiert.

Why is "diskutiert" placed at the very end of the sentence?

In German, the spoken past tense (the Perfekt) creates a "verb bracket". The helping verb (in this case, haben) takes the normal second position in the sentence, while the past participle (diskutiert) gets pushed to the very end. Extra information, like das Projekt, gets sandwiched in between.

Is "das Projekt" in the accusative case here?

Yes, it is! The project is the direct object, meaning it is the thing receiving the action of being discussed. Because Projekt is a neuter noun, its accusative form (das) looks exactly the same as its basic nominative form.

Why do we use "haben" instead of "hat" as the helping verb?

The helping verb must match the subject of the sentence. Since the subject is Anna und Peter, it counts as a plural "they". The plural form is haben. If it were just Anna, the sentence would use the singular form: Anna hat das Projekt diskutiert.