1. We was playing tennis when it were starting to rain. Choices: were playing / played; was starting / started.

Перевод задания: Простое прошедшее или прошедшее длительное время.

We were playing tennis when it started to rain.
Мы играли в теннис, когда начался дождь.

Длительное действие прерывается коротким событием: were playing — Past Continuous, started — Past Simple.