我想问一下,恩,过去式的用法.就是说,比如:,像I started __________ (play/playing)

我想问一下,恩,过去式的用法.
就是说,比如:,像I started __________ (play/playing) football when I was nine years old.应该填play呢?还是playing
还有:When my mother was shopping,she heard many people _______ about the robbery.应该填talk 还是talked 还是talking?
为什么要这样写呢?有什么公式吗?(我指的公式像:see sb doing sth begin doing sth 之类的)