Certainly the primary duty of a defender is to defend. However, when your team has lost possession of the ball, everyone must then think and act defensively to regain possession.
Diagram 1. Basically, there are four defenders who operate along the back line of a team. The two fullbacks mark the opposing wingers and the center back marks the striker. The remaining defender may be the sweeper. Variations can be applied, but, remember, the opposing forwards must be marked closely most of the time.
It is important to have an understanding with the goalkeeper. He should be your best adviser throughout the game.
Many coaches feel that defenders must be big and strong. I can agree if they are also agile. The central defenders are expected to win the majority of head balls, so weight should be an advantage there.
Diagram 2. Do not stand square to an oncoming forward; rather, show him a "path" by positioning your body to one side and shepherding him away from the danger area.
Good anticipation can help intercept passes, but if your opponent does secure the ball with his back to you, do not allow him to turn.
"Niggle" him and force him to play the ball backward. If and when a tackle has to be made, be quick and positive in your technique. Do not attempt to dribble past opponents in the defensive part of the field. Defenders are expected to have good kicking skills with both feet, often for clearance kicks but also for passing to teammates.
Diagram 3. Putting on cover must be learned. Generally, the farther you are from the ball, the farther you can mark your opponent. Cover must then be applied.
Player 01 has the ball, defender X1 is in close contact. The gap between defender and attackers widens until we can see X4 preparing to cover the possible danger area in front of goal, but also ready to go closer to 04 if necessary. There are times when it becomes more important to cover space rather than your opponent.
Occasionally, defenders may seize an opportunity and advance upfield into an attacking position. In these instances, teammates must be prepared to fill in for the defender if the forward move breaks down.
On set plays, for example goal kicks and free kicks in the opposing half, do not hesitate to send defenders, who are good in the air, into th epenalty area. They will get their share of goals.