There was илиr there were a lot of people in the theatre yesterday

Правильно: There were a lot of people in the theatre yesterday.

Почему так:

• A lot of people = много людей. Это множественное число, потому что people — это множественное (нет формы peoples в обычном значении “люди”).

• С оборотом there is / there are в прошедшем времени мы используем:

  • there was — для единственного числа (a person, a man, a lot of water и т.п.)

  • there were — для множественного числа (people, students, chairs и т.п.)

• Поэтому с people нужно were.

Примеры для сравнения:

• There was a lot of noise in the theatre yesterday. (noise — неисчисляемое, “шум”)

• There were a lot of people in the theatre yesterday. (people — множественное число)

Итог: There were a lot of people in the theatre yesterday.