1) the thieves were running / ran away when they were hearing / heard the police sirens. 2)the police were catching / caught the burglar as he escaped / was escaping from the back door. 3)the old woman was screaming / screamed when she was seeing / saw a stranger in her house.
2) The police caught the burglar as he was escaping from the back door.
3) The old woman screamed when she saw a stranger in her house.