We were having supper when they ________. Варианты: are arriving; arrives; were arriving; arrived.

We were having supper when they arrived.

Используется Past Simple (arrived) для краткого действия, которое произошло во время другого действия, выраженного Past Continuous (were having). Перевод: «Мы ужинали, когда они приехали».

Ответ: arrived.