1.  Tommy's mother often (tell) him that he (spend) too much money on running shoes but he never (listen).

Tommy's mother often TOLD him that he WAS SPENDING / SPENT too much money on running shoes but he never LISTENED.

2.  As Mr. Wells (cross) the street he (step) on a banana skin and (fall) heavily.

As Mr. Wells WAS CROSSING / CROSSED the street he STEPPED on a banana skin and FELL heavily.

3.  The policeman (clean) his gun when it accidentally (go) off and (kill) him.

The policeman WAS CLEANING his gun when it accidentally WENT off and KILLED him.

4.  The thief (open) the safe when he (hear) someone coming, so he immediately (crawl) under the bed.

The thief WAS OPENING the safe when he HEARD someone coming, so he immediately CRAWLED under the bed.

5.  The president (play) chess when he (receive) the news of the war, but he (insist) on finishing the game.

The president WAS PLAYING chess when he RECEIVED the news of the war, but he INSISTED on finishing the game.