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Задание 3:

When I arrived at the party, everyone had already left.
She had finished her homework before she went out to play.
After he had eaten all the cookies, he felt sick.
When I got to the cinema, the film had already started.
They had studied for hours before the exam.
I met him yesterday, but I had never seen him before.
She had bought a new dress before she went to the party.
She had read the book twice before she wrote the essay.
By the time he arrived at the meeting, it had already started.
Задание 4:

She had already eaten breakfast when he called.
We had finished working by five o'clock.
When I came home, my parents had already left.
He had never flown on a plane before that moment.
They had lived in this city for five years when they moved.
I had lost my keys before I found them under the sofa.
By the time the teacher arrived, the students had already left.
We had already packed our bags before the taxi arrived.