Into is for moving to the inside of something or changing from one state to another, while onto is for moving to a position on top of something. Picture a ball rolling into a box versus a ball landing onto a shelf, and the difference becomes clear.
The core difference
Into is about entering or transforming; onto is about landing on top of a surface.
- into — moving toward the inside of something: She walked into the room and switched on the light.
- into — changing from one form or state to another: The caterpillar turned into a butterfly.
- onto — moving to a position on top of or upon something: The cat jumped onto the windowsill.
How to tell them apart
The clearest test is to ask where the motion ends up: into ends inside something, while onto ends on top of something. "Into the box" means the object is now enclosed; "onto the box" means it's now sitting on its surface.
Both words are formed the same way, from "in" or "on" plus "to," which is exactly why they get mixed up so often — they look like a matched pair even though the destinations they describe are different. Informally, into also covers being interested in something ("he's really into old vinyl records"), while onto can mean being aware of a secret ("the police were onto the smuggling ring"), and neither of those figurative uses crosses over to the other word.
