Помогите пожалуйста .Раскройте скобки , употребляя глаголы в Present Simple или Past Simple:the lesson already (to start) when (to arrive).When Christina(to arrive ,the film ... already.(to begin).He ..(to play )guitar before he..(to sing)a song.children ..(to ride) their bicycles before they ...(to meet)friends

The lesson had already started when we arrived.
When Christina arrived, the film had already begun.
He had been playing  the guitar before he sang a song.
The children had been riding their bicycles before they met friends.