In, into, on and onto are all prepositions. We usually use 'in' and 'on' to give information about position of the subject in relation to something else. For example:
- He splashed in the puddles.
- She stood on the wall.
- He raced into the lead.
- She climbed onto the wall.
- She turned him into a frog.
- The boy jumped in the pool.
- The boy jumped into the pool.
A further complication arises when the verb in a sentence is a phrasal or prepositional verb. Many verbs are constructed by using a verb and a preposition: hand in, turn in, give in, build on, rely on, etc. When these are followed by 'to', we really need to think about the sense we are creating for the reader. Joining the final preposition of the verb phrase to what follows may change the meaning altogether if care is not taken. Consider the following sentences:
- My homework should be handed in to my teacher by Thursday.
- My homework should be handed into my teacher by Thursday. (Not possible - the adverbial phrase 'to my teacher' cannot be joined to the 'in'.)
- We need to turn ourselves in to the police.
- We need to turn ourselves into the police.
- I'll give in to keep the peace. (Correct as the 'to' starts an infinitive verb structure and non-finite clause)
- I'll give into keep the peace. (Not possible)
- Her success can be built on to ensure her future. (Correct - the non-finite clause 'to ensure her future' has to be separate.)
- Her success can be built onto ensure her future. (Not possible)
- His father could be relied on to keep him safe. (Correct - the non-finite clause must be separate.)
- His father could be relied onto keep him safe. (Not possible).