You can play a trump only when you cannot follow the suit that was led, because following suit always comes first. If you hold a card of the led suit, you must play it. If you are void in that suit, you then have the option to trump, which usually wins the trick unless someone over-trumps you. When a trump is led, though, trump becomes the required suit for that trick, so everyone with a trump must play one. And remember the over-trump rule: if a trump is already in the trick and you trump too, you must beat it if you can.