How to use "useful" in a sentence

What more can I do? No voice inside my head suggests anything useful

'This money will be useful,'Thenardier said to his wife

The tall, grey-haired man was doing useful work on the larger barricade, and Eponine (whom everyone thought was a boy) worked hard too

I can see that all those units and the extra money would be useful

"Have we learned anything useful?"

"I'm not sure if it's useful or not." He frowned

And he thought that it might be useful, so he put it in his pocket.