The boy heard the engine, and he raised his head.

The boys could have heard the shooting, but not many of them, if any, were up and about.

The boy must have heard what happened to the woman.

The boys heard the sound of a car on the road.

If they did not keep their voices down, the boy would hear.

The boy heard him passing close by, but did not answer.

The boy would have heard all the stories, of course.

One of them was speaking, but the boy could hear nothing.

The boys heard a car on the road behind them.

No response; he wasn't sure whether the boy had even heard him or not.