Prepositions are words or group of words placed before nouns to indicate the relations to another noun or pronoun.
Prepositions limit the meaning of the noun or pronoun placed after them.
Look at these pictures.

       
   The pen                          The box

Now, look at this picture and see the relations between two nouns.

     
          The pen is on the box

———————————————————————————————————————————————

Read these sentences and follow the picture.

The light is over the table.
The flowers are in the vase.
The cup is near the vase.
The glass is in front of the box.
The bottle is behind the box.
The knife is by the box.
The pen is on the box.
The chair is at the table.
The dog is under the table.
The old woman is looking at the road through the window.
The picture is on the wall between the door and the window.
The clock is over the door.
The woman is at the door.
The little girl is leaning against the woman.
The man is coming into the house.
The cat is going out of the house.
The boy is going up the steps.
The girl is coming down the steps.
The baby is playing with a toy car.
There is an old man at the bus-stop.
There is a boy running along the road.
There is a girl walking across the road.