Although originally two words, ‘on’ and ‘to’ are now commonly combined into ‘onto’ when used as a preposition. Prepositions specify relationships between other words in a sentence, with ‘onto’ meaning ‘to position upon’ or ‘on top of’:
The dog jumped onto the sofa.
Informally, ‘onto’ is also used to suggest someone is aware of something:
Donald thinks he’s got away with it, but the FBI are onto him now.
Generally, it’s only the ‘to position upon’ sense of ‘onto’ you’ll need in formal academic writing.
On To (As Part of a Phrasal Verb)
Although many treat ‘onto’ as if it can be used anywhere, the words ‘on’ and ‘to’ can’t always be combined. When ‘on’ is part of a phrasal verb followed by ‘to’, for example, they should be kept separate:
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You need to pass your exams before you can go on to university.
In the above, ‘on’ is part of the phrasal verb ‘go on’, not the preposition ‘on to’. If ‘on’ was part of the preposition here, it would suggest you have to pass an exam before climbing on top of a university!
Onto or On To?
The important thing is to remember ‘onto’ is a preposition, whereas ‘on to’ is usually part of a verb phrase. This difference is vital, as you shouldn’t use ‘onto’ other than as a preposition if you want to ensure clarity.
For example, after finishing your main course at a restaurant, saying you’re ‘moving on to dessert’ simply means you’re going to start the next course. This is an entirely reasonable thing to say.
By comparison, saying you’re ‘moving onto dessert’ would imply you’re going to sit in a bowl of ice-cream, which at best promises a soggy bottom and may get you kicked out of the restaurant.