Learn Italian: Credevo che la temperatura fosse scesa a venti gradi. - I believed that the temperature had dropped to twenty degrees.

We use the pluperfect subjunctive to show that an action happened before a past action in the main clause. Here, the temperature dropping happened before the believing (credevo). If the drop was happening at the exact same time you were believing it, you would use the imperfect subjunctive (scendesse).

Why is the auxiliary verb fosse instead of avesse?

The verb scendere (to drop or go down) is a verb of motion or state change. In Italian, these types of verbs require essere as their auxiliary in compound tenses. The imperfect subjunctive of essere for the third person singular is fosse.

Notice how the word is scesa and not sceso. Why does the ending change?

Whenever a compound tense uses essere as its auxiliary, the past participle must agree in gender and number with the subject. Since the subject is la temperatura (feminine singular), the participle sceso changes to scesa.

Could I just use the normal past tense and say credevo che la temperatura era scesa?