“into” vs “in to” and “onto” vs. “on to”

Errors like this are becoming more prevalent for sure. I see them everyday (sic). It maybe (sic) because people are exposed to a lot more non-proof-read text (such as online), and they get their ideas of what is and isn't correct English from that, so we end up in a vicious circle.

I haven't seen confusion between preposition 'to' and infinitive 'to', but as Brus says, that should be easy enough to explain (provided people know what a to-infinitive is).

I think Brus's second type of example - 'he carried on to Rome' - is more likely to cause confusion, because here we have two prepositions, and I'm pretty sure I've seen this happening. I think we would also keep the two prepositions separate in these examples:

'Let's pass on to the next subject'
'He is slow at catching (or cottoning, or latching) on to new ideas'
"You need to hand it in to the teacher'
'When you've finished with it, hand it on to the next person'
'We need to keep on to the end of this road'

As foreign learners will well know (but native speakers might well not) these are all phrasal verbs, in these cases, specifically prepositional verbs. The verb in 'he carried on to Rome' isn't 'carry' (what did he carry?), but 'carry on'. That's one way of explaining it.

The other is that 'in' and 'on' here aren't about physical position or physical movement - when you hand something 'on to' somebody or 'in to' somebody, you don't physically put it 'onto' them or 'into' to them, we are using 'on' and 'in' metaphorically here.

You can find lists of these verbs at Wiktionary:

http://en.wiktionary.org/wiki/Category:English_phrasal_verbs_with_particle_%28in%29
http://en.wiktionary.org/wiki/Category:English_phrasal_verbs_with_particle_%28on%29

But sometimes similar looking verbs have a literal meaning as well as a metaphorical one. For example 'lead on to' is sometimes used to mean 'result in'. Macmillan Dictionary gives this example - 'Prout's work on digestion led on to studies of proteins and fats.'

But we can also say 'The French windows in the living room lead onto the garden', where there is a physical meaning. At a British local government website there is a piece headed:

'Launch success leads onto Streetwise training'
(http://www.suffolkcoastal.gov.uk/news/launch-success-leads-onto-streetwise-training/)

Basically, the launch of a new campaign was so successful that the council are rolling out 'Streetwise' training sessions. In this case, there is no physical meaning, and 'leads on to' is a three-part phrasal verb meaning 'has resulted in', so I personally would write 'leads on to', as in this example from the NYT:

'her shrewd and happy argument that a generous policy of tolerance and inclusion leads on to success and prosperity.'

Even more interesting(ly), when you Google 'leads on to better things', it asks you you if you mean 'leads onto better things', which for me is not correct. Actual figures on Google are:

lead on to better things - 121 - lead onto better things - 65
leads on to better things - 48 - leads onto better things - 37
led on to better things - 30 - led onto better things - 15

At Google books 'leads on to success' gets about 15 hits, 'leads onto success' just one. There are similar results for 'leads on to (onto) better' with only one for the latter, although interestingly it's from 1912, so this is hardly a new phenomenon. Its an advertisement for 'The Early Educator' :

'If studiously followed, leads onto better returns for efforts exercised'

I'm minded (reminded) of the famous (for older people in the UK, and fans of Birmingham City Football Club) Harry Lauder WW1 music hall song - 'Keep Right on to the End of the Road'.

'https://www.google.com/search?q=%22keep%20right%20on%20to%20the%20end%20of%20the%20road%22'

But of course it turns out that there's at least one chump (a Scotsman who should know better - Lauder was Scots) who has recorded it as 'Keep Right onto the End of the Road' - but nobody's really putting anything 'onto' the end of the road, are they?

Funny you should mention that WW.
As it happens I lived in Birmingham for three years and saw Blues play a number of times. Yes it's definitely 'on to' as two separate words. In the song there's an audible pause between 'on' and 'to', supporters often abbreviate it to "Keep Right On" and, as you say, 'onto' doesn't make a lot of sense there.
I still check their results from 12,000 miles away. Big win over Millwall this morning (my time).

Wow - impressive knowledge and explanations there. I would definitely write "lead on to better things" too, although I wouldn't have been able to properly explain why.

I note that some people have an aversion to 'onto' and never use it (I'm not one of those people).

I should also mention the relatively new "please login". You can just about get away with 'login' as a noun (i.e. username and password), but as a verb I think it needs to be two words.

My previous comment was more of a general one about native speakers not knowing whether to use one word or two in various phrases, and I think a lot of them (us) don't care either.

@Chris B - I think you're absolutely right about 'everyday' and 'maybe'; you see these sort of things quite a lot in comments columns. And I quite agree that this is probably more of a native-speaker problem, like 'their / they're / there', 'your / you're' and 'must of' for 'must have'. Native-speaker grammar sites are full of warnings about these, but I haven't noticed foreign learners make the same mistakes.

Mind you, we all make typos now and then, and I seem to remember being pulled up at least once in these pages for a stray 'it's' masquerading as an 'its'. But that is more to do with carelessness than ignorance, and most of us are guilty of that from time to time.

Correction - I've just realised that I made one basic mistake in my explanation. In the examples I gave, there weren't two prepositions, with expressions like 'keep on to', 'hand in to' the first particle is an adverb, not a preposition, and these verbs are simply phrasal verbs, not prepositional verbs, an example of which would be 'look for' or 'deal with'. Although in modern parlance, three-part phrasal verbs like 'lead on to' are sometimes referred to as phrasal-prepositional verbs.