So I'm still editing those old published manuscripts, and am now looking at 'into' vs. 'in to.' I found this:
Use 'in to' if you can generally replace it with 'in order to.' Use 'into' if it means or can answer the question 'where' or indicates some sort of action.
Personally, I think of 'in to' as 'inside to'. I think of 'into' as just 'inside.'
I walked into the room. (where? the room. OR I walked inside the room. Use into.)
I went in to put the book away. (I went, in order to put the book away, OR I went inside to put the book away. Use 'in to'.)
Easy, peasy. YAY!
Here are a few references: