the preposition form of ‘onto’ is used to indicate movement in the direction of, or placement on top of, another object. It is a compound preposition formed by combining ‘on’ and ‘to’, and specifically implies movement onto or placement onto something, whereas ‘on’ does not. For example, you could say 'she stepped onto the train’, or 'she put the pot onto the stove'.
1. We hopped onto the wagon.
2. The policeman dragged her onto the sidewalk.
when using ‘onto’ in its preposition form, always place it before the object onto which an action is directed. Do not use it interchangeably with ‘on’, as the two can hold different meanings, such as ‘onto’ suggesting movement or placement onto something, while ‘on’ implies a position or placement.