Warding plays a crucial part in control of the map.
In the picture above, dots represent common warding spots. The white rectangles show camp spawn trigger locations. If a unit or ward is in it at the spawn times, it will not spawn.
Where you place wards depends on the stage of the game. In the early phase, you place a ward for the river rune and a ward for your safe lane. It is sometimes smart to place a sentry ward on the opponent's side to allow your mid player to see smoke ganks.
In an offensive tri-lane or when pushing a tower, an observer ward placed behind the tower allows you to see the movements of enemy heroes.
In the later game stages when you are playing offensively and keeping lanes pushed, river wards are no longer the best. Instead, wards should be placed on the enemy's side of the map.
The fight for control of Roshan vision often leads to warding battles.
By learning and avoiding the most common ward spots and understanding the purpose of wards, you can find spots that opponent's are less likely to auto-deward.
Sentries are also critical for spotting enemy smokes and invisible heroes.
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