9. He … (swim) for 15 minutes when he understood he had lost his swimsuit.

Перевод задания: Упражнения на Past Perfect и Past Perfect Continuous: заполните пропуски подходящими формами глаголов.

He had been swimming for 15 minutes when he understood he had lost his swimsuit.

Он плавал уже пятнадцать минут, когда понял, что потерял купальный костюм.