Prepositions of
      Movement, Destination & Position




into

... to signify movement towards the inside of something

Example: Beannie is inserting the graded assignments into the slots.

onto

... to signify movement towards the surface of something

Example: Beannie is jumping onto the table.


After some verbs (e.g., dive, jump, push, put, throw), in and into are interchangeble; so are on and onto. However, it is preferable to use into/onto when we talk about movement and destination; use in/on for where somebody or something will be (i.e., position) at the end of the movement.
  Movement & Destination   Position at the end of movement


Beannie dived into the water.


Beannie is in the water.


Beannie fell onto the grass.


Beannie is on the grass.


to, toward & towards


After verbs of movement (e.g., move, drive, fly, go, ride, run, swim, walk), we can use both to and toward (towards) to talk about movement.
But note the difference in meanings.