Заполните пропуски: Why ... late yesterday? Варианты: 1) you was late; 2) you lated; 3) were you; 4) you were.

Правильный вариант: were you.

Полное предложение: Why were you late yesterday? — «Почему ты опоздал(а) вчера?» В вопросе в прошедшем времени используется обратный порядок слов: were + подлежащее.

Ответ: were you