Use an older card on your own turn
A Knight bought this turn is not playable yet. On a later turn, you can play it before rolling or during the action phase, provided you have not already played another action development card that turn.
Playing before the roll can unblock your production before dice are resolved. Playing after the roll lets you use the new information about payouts and current hands. Neither timing guarantees a better result; choose according to the board.
Move to a different tile
The robber must leave its current tile. Its destination stops producing resources while occupied. If several players touch the target tile, the block affects all of their adjacent buildings, including yours if you share that tile.
Look beyond the number alone. Blocking a grain tile that supplies an opponent’s next city may matter more than blocking a higher-frequency wool tile they cannot currently use. Your own production loss belongs in the same calculation.
Steal only from an eligible neighbour
An eligible victim has a settlement or city beside the new robber tile, is not you, and holds at least one resource card. The theft takes one random resource from that player; you do not select its type.
If there is no eligible victim, moving the robber can still be useful because of the production block or the tile you freed. A large hand offers more possibilities but does not promise the particular ore you need.
Count the Knight, not a discard
Playing the Knight increases your played-Knight total and can help claim Largest Army. It does not cause everyone with more than seven cards to discard. Discarding is triggered by a dice total of seven, not by every robber move.
Before playing, check whether the card completes a qualifying army lead or protects an important upcoming roll. Those are concrete benefits you can assess without guessing opponents’ private cards or a future dice result.