When should I use 'in between' and 'between' ?Is there a difference between them?
'Between' functions as a prepostion:
I used to live in Fareham, between Portsmouth and Southampton.
I'll see you tomorrow between seven and eight.
'In between' functions adverbially:
Mila's house is not far from mine, but there is a railway line in between.
We go to the pub after our Thursday tutorials, but we don't meet in between.
As 5jj's examples show, "in between" can come at the end of a sentence. "Between" can't.
"in between" can be used as a compound preposition, as can "in among", and various others.
"The penny had fallen in between the cushions."
"I found the shell in among the rocks."
In these cases, there is an element on "in" as well as "between/among".
The following lists "in between" as "prep. & adv"
http://www.thefreedictionary.com/in+between