Is there a difference between 'on to' and 'onto'; and 'in to' and 'into'?
In this sentence, "We didn't want to get 'in' trouble", the word 'in' can be placed with 'into'? Is these words interchangeable?
Another questions "the sticker can be placed onto the mirror", is the word "onto" sounds right?
Can anyone please let me know?
Hello Joeviee

Your questions are a bit too tedious to answer in detail. But let me try it. I think we'd better divide your first question into two parts.
Is there a difference between 'in to' and 'into'?

In old days (say before 900 AD), the preposition 'in' was used in two ways; [1] a static/location 'in': (EX) I slept in the house, and [2] a dynamic/motion 'in' : (EX) I went in the house. These two 'in' were distinguished by the case of the noun that followed the preposition. But as the time passed, case-endings gradually got weakened and people felt the need of some means to make distinctions between the dynamic 'in' from the static 'in'. So people invented a new preposition 'into' to substitute the dynamic 'in'. Basically, in modern English, we can take 'in' as a preposition to indicate a location [at a place inside of], whereas 'into' as a preposition to indicate a direction [towards a place inside of]. But still now many phrasal verbs consist of a verb and the dynamic 'in'. For example, 'go in' is an intransitive phrasal verb to mean 'get in' or 'go inside'. If you say "The knight went into the room", you conceive only one space, the room. But when you say "The knight went in to the king", you conceive two spaces : a room the king was in and a place the king sat in. That is, the sentence means "The knight went in the room and walked to the king".
Is there a difference between 'on to' and 'onto'?

The relation between 'on' and 'onto' is similar to that between 'in' and 'into'. "The boy jumped on the table" is someway ambiguous. It could mean either "The boy did the action of jumping on the surface of the table" or "The boy jump up to the surface of the table". If you say "The boy jumped onto the table", you can wipe that ambiguity. What makes the matter a bit complicated is that even now some people write 'on to' to mean 'onto'. It is because 'onto' was a relatively recent invention compared with 'into'. ('Into' was invented around 1000 AD while 'onto' around 1700 AD). Beside this problem, you should know the alignment of 'on to' quite often occurs as a combination of a phrasal verb and the preposition 'to'. For example, "walk on to the next station", "flow on to the sea", "hang on to a party", "lead on to another point", etc.
"We didn't want to get 'in' trouble", the word 'in' can be placed with 'into'?

I would say rather 'get into trouble', though 'I got in trouble' could stand as an informal substitute for 'I was in trouble'.
"The sticker can be placed onto the mirror", is the word "onto" sounds right?

Because 'be placed' sounds a stative predication, I would say "The sticker can be placed on the mirror"

I'm a mere learner as you might know, and it is quite possible I made some mistakes in this reply. Please wait until our moderators give you better answers.

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Hi Paco,
Thanks Paco. I really admire your extensive knowledge in english language eventhough this is not your first language. I have been learning english for a rather long time but i can't seem to master it well. I am hopeful i can benefit from this forum with the helps of others.
Hello Joeviee

Thank you for the compliment. But I'm still bad at writing English. I think I have to continue the learning more. Anyway thank you for the reply.