ONTO: position on top of, implies movement. It is a preposition:
"The dog jumped onto the box." 'onto the box' is a prepositional phrase.
ON: is a specific position physically in contact with a surface So these two, onto and on can be used interchangeably but not all the time.
"There's a tatoo on her arm." This shows position - no movement.
ON TO: is an action of the verb "She held on to the rope while jumping." 'she held on' - 'to the rope' is the prepositional phrase.
A way to remember on to vs. onto is to say “up” before on in a sentence. if "She held 'up' on to the rope." Don't use on to.
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