Fill in the blanks with the correct form of the verbs. 1- He ................................. (play) the piano when I called him.

Для образования Past Continuous необходимо использовать вспомогательный глагол to be в прошедшем времени (was/were) и основной глагол с окончанием -ing.

В данном случае подлежащее - He, следовательно, используем was.

play + -ing = playing

Получаем: He was playing the piano when I called him.

Ответ: He was playing the piano when I called him.