When to use "in to" and "into"?



Hello! This is a great question, and something that many people get confused with.
'into' is a preposition that is used to indicate movement, direction, entry, insertion, collision or transformation. For example, 'I walked into the room'.
'in to' is used when 'in' is part of a phrasal verb or 'to' is part of an infinitive. For example, 'I went in to see my aunt'. The words 'to see' indicate an infinitive.
Sometimes, 'into' and 'in to' can be used interchangeably. For example, 'I logged into my account' and 'I logged in to my account'. However, in most cases it would be confusing to do this, such as in your example, 'He turned the documents in to the office'. If we used 'into', it would mean that the documents had transformed, and become the office. Instead, break the sentence up: 'He turned the documents in.' is one sentence. Where did he turn the documents in: 'To the office'. That's the second part which indicates location.
In your first example, 'He dove into the pool'; 'into' indicates movement, entry and direction.
I hope that this helps, and makes the use of 'into' and 'in to' less confusing.