Rule 1: In general, use onto as one word to mean “on top of,” “to a position on,” “upon.”
He climbed onto the roof.
Let’s step onto the dance floor.
Rule 2: Use onto when you mean “fully aware of,” “informed about.”
I’m onto your scheme.
We canceled Julia’s surprise party when we realized she was onto our plan.
Rule 3: Use on to, two words, when on is part of the verb.
We canceled Julia’s surprise party when we realized she caught on to our plan. (caught on is a phrasal verb)
I’m going to log on to the computer. (log on is a phrasal verb)
Here’s a little quiz to see if you understand the difference between onto and on to:Click here