Using For

According to Martinet (1985: 290) For is used to express causal relation – one element is a cause of the other. Examples:

1.      He ran away, for he was frightened by what he saw.

2.      The days were short, for it was now December.

3.      He took the food eagerly, for he had eaten nothing since dawn.

4.      When I saw her in the river I was frightened. For at that point the currents were dangerous.

5.      The boy keeps the light on, for he is afraid of sleeping in the dark

In speech a short pause is usually made before a For and in written English this place is usually marked by a comma, and sometimes as in the last example above by a pull stop.