I play (bad) chess with my grandsons. They say that to touch a piece means that you MUST move that piece. Their chess club teacher tells them so. I was taught that a piece isn't considered moved until you stop touching it. My chess club master told me so, many years ago. Who is right?
You are both right! If you touch a piece you MUST move it. But the move is executed as soon as you stop touching the piece.
For example: If you move a knight to e4 then this move is executed when you stop touching the knight.
When you touch a piece you must move it somewhere.
When this is done and you still touch the piece you can move the piece somewhere else from where you have placed it before, because you have not stopped touching it. You didn't let go of the piece. Read more about Touch Move Rule at Wikipedia