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 play )guitar before sing)a song. children ride) their bicycles before they 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.