Pick up cards and drop them
You play your cards by picking them up, and then drop them at another place.
Pick up and drop (by clicking)
You can pick up any card that is highlighted when you move the mouse over it, with a simple click.
Now the card is attached to your mouse, and you can move the card to where you want to play it. If you move the card over other cards on the table, you'll see an insertion mark at the place where the card would go. (You won't see an insetion mark before you drop a card on an empty place on the table.)
To drop a card, simply click again.
Pick up and drop (by dragging)
A slightly faster method to play a card is to play it by dragging. Click the card, but hold the mouse button depressed while you move the card. Then drop the card by releasing the mouse button.
Using the keyboard
As an alternative to clicking the mouse, you can hit the spacebar to pick up a card or drop it.