Learn Italian: Ho letto le notizie perché pensavo che le previsioni fossero cambiate. - I read the news because I thought that the forecasts had changed.

The trigger is the verb phrase pensavo che (I thought that). Verbs expressing personal opinions, beliefs, or uncertainties require the subjunctive mood in the clause that follows them.

Why use the pluperfect subjunctive (fossero cambiate) instead of the imperfect subjunctive (cambiassero)?

We use the pluperfect (past perfect) subjunctive because the action of the forecasts changing happened before the action of thinking about them. Since the main verb (pensavo) is in the past, and the changing happened even further in the past, we need this past-in-the-past subjunctive form.

Why does the word cambiate end in -e?

Because the helper (auxiliary) verb is a form of essere (fossero). In Italian, whenever a compound tense uses essere, the past participle has to match the subject in gender and number. The subject is le previsioni (feminine plural), so cambiato changes to cambiate.

I have seen cambiare used with avere. Why does it use essere (fossero) here?