There wasn't much traffic in the street.
I saw a little girl CROSS the road.
Question: Why "cross"? Why not "crossing"? Please, explain the rule or the sentence if it is correct.
"I saw her cross the street" describes the event as a complete action from start to finish, while "I saw her crossing the street" describes the action as something that was in progress when you observed it. The first emphasizes what she did, the second emphasizes the girl's state at the time you saw her.