Answer:
grübelte; gesehen und gehört hatte.
Explanation:
This is a complex sentence; it contains an independent clause and a dependent clause. The verbs in the independent clause are ging and grübelte. Since the first verb (ging) is in the Präteritum, the second verb should be used in the Präteritum, too, because the independent clause shows a series of actions in the past. Grübeln is a weak, or regular, verb, and its Präteritum form is also regular: grübelte.
The verbs in the dependent clause should be used in the Plusquamperfekt (the past perfect tense) to express actions that occurred in the past and finished before the actions in the main clause started. You make the Plusquamperfekt forms by using the imperfect verb form of the auxiliary ‘haben’ (for transitive verbs, such as sehen and hören) and the past participle of the full verb.
Sehen is a strong, or irregular, verb, and its past participle form is also irregular: gesehen.
Hören is a weak, or regular, verb, and its past participle form is also regular: gehört.
"Ich ging langsam nach Hause und grübelte über alles, was ich gesehen und gehört hatte."
