10. He (think) that we (already, drop) our parents off at the station.

10. He thought that we had already dropped our parents off at the station.

He thought - Past Simple, действие в прошлом. we had already dropped - Past Perfect, действие, предшествовавшее моменту, когда он подумал об этом.

Ответ: He thought that we had already dropped our parents off at the station.