Вот правильные времена для глаголов в ваших предложениях:
When I arrived at the party, everyone was dancing and having a great time.
She has not seen him since they graduated from university.
They had been playing football for two hours when it started to rain.
He finished his homework before he went out to play.
While I was walking in the park, I met an old friend I had not seen for years.
By the time we got to the cinema, the movie had already started.
She had studied English for five years before she moved to London.
I tried to call you last night, but you did not answer.
They had been waiting for the bus for half an hour when it finally arrived.
After he completed the project, he felt a great sense of accomplishment.