When I rang up my friend he was sleeping.
When grandfather watches TV he was falling asleep.
When my friend came to see me I was doing my hw.
When I went to the stadium I was meeting Kate and Ann.
When Nick rang me up yesterday I was helping mother.
When the children walked through the wood they were seeing a fox.
When I came home my sister was washing the floor.
И так далее после when ставь past simple, во второй части этого сложного предложения present continuous. Я написала кучу образцов
12.I went to the theatre
13. went
14.did; played
16. played
18. went
19. did you go to Great Britain last year? No I went to France
20. did; translated