Use one-word onto for landing on a surface: The cat jumped onto the table. Use on to when on belongs to the verb: He carried on to the next task. Quick check: can you stop at on? She moved on. works, so two words.
When the sentence describes landing on a surface
Quick test before you write: say the sentence aloud. If it ends naturally at on, the on belongs to the verb, and to is written separately as on to. If not, choose one-word onto.
If someone or something lands on a surface, use one-word onto: She stepped onto the platform just as the train arrived. If you can slip up in there, stepped up onto the platform, the one-word form is safe.
Same for things ending up on top of something: Pour the sauce onto the pasta and serve it straight away. The sauce finishes its journey resting on the pasta, so onto reads naturally.
Now the contrasting case: when on belongs to the verb, split it. We logged on to the new system this morning. Here logged on stands on its own, so one-word onto would garble the phrasal verb.
A quick edge check: some sentences allow either, especially with movement like held on to or held onto the railing. If you are unsure, ask whether on could end the clause. She held on. works, so the two-word version is never wrong there.
You chose onto for The cat jumped ___ the counter., and that is the form that fits. Onto works here because the cat lands physically on a surface, on the counter itself, rather than moving onward to something else. When verbs like carried on appear, as in carried on to the next experiment, the on belongs to the phrasal verb, so the two-word on to is the right spelling in those sentences instead.
When on belongs to the verb
The condition is whether on belongs to the verb in front of it. With phrasal verbs like catch on or go on, write two words: She caught on to the game in one round. One-word onto there snaps the verb apart.
Test it by stopping at the verb. If She caught on. makes sense as a complete sentence, on belongs to caught, and whatever follows takes a plain to: She caught on to the game. If the clause cannot end at on, pick one-word onto, as in He leapt onto the stage. One honest caveat: with movement verbs like leap, careful writers sometimes accept the joined form even when the stop test points the other way, so neither choice is flatly wrong there. If you want a rule that never misfires, keep the two-word version whenever the clause could stop at on.
One honest caveat: with movement verbs, careful writers sometimes accept the joined form, and neither choice is flatly wrong there. If you want a rule that never misfires, keep the two-word version whenever the clause could stop at on.
He carried ___ to the next experiment shows the stop-at-the-verb test in action. Here carried on is a phrasal verb, so the clause can end right at on: he carried on. Whatever comes next takes a plain to, giving He carried on to the next experiment. Because on belongs to the verb, the two-word spelling is the one that fits.
On to is right here, because carried on is the complete verb and stands on its own, so it takes a plain to after it.
Careful: if you write onto as one word, you weld the preposition to the verb and garble carried on. The verb here is carried on on its own, so apply the stop-at-the-verb test to the same sentence and see where it leaves you.