Earlier in the past: Plusquamperfekt
For an event before another past event, use hatte or war plus Partizip II. Keep the same auxiliary as in Perfekt. The finite auxiliary goes last in a subordinate clause.
Understand the idea
Plusquamperfekt looks back from a past reference point to an earlier event. It is useful when the order of past events matters. The earlier event uses hatte or war plus a participle; the reference event may be in Präteritum or another appropriate past tense.
Build it step by step
Choose the same auxiliary that the verb uses in Perfekt, then put that auxiliary in Präteritum. Conjugate hatte/war for the subject and leave the participle unchanged. In a basic subordinate clause, place the finite auxiliary after the participle. Establish the past reference point before deciding that an event needs this earlier-past form.
See it in a sentence
Als ich ankam, hatte der Film schon begonnen.
Wir hatten schon gegessen, als sie anrief.
A useful memory aid
Draw two marks on a timeline: earlier event, later past event. Attach Plusquamperfekt to the earlier one.
A common trap
Do not choose hatte for every verb. A verb using sein in Perfekt normally keeps sein as war in Plusquamperfekt.