"He lives between us."
Translation:Hij woont tussen ons in.
There's an explanation about Dutch prepositions here.
As a native English speaker, from what I gather "in" is used like "in" in English. It's not mandatory but I think if you want to add emphasis that the subject is in between the object, you can add it :)
- Zij staat tussen de auto en de tuin in.
She is in between the car and the garden
Here, you're saying that she is specifically in between the car and the garden. Whereas if you drop the in, it's roughly saying she is somewhere between the two. This is what I gather from having been learning Dutch for quite a while now, so please don't quote me on it as I'm not a native but it's my interpretation (I still recommend reading the explanation that I have linked about Dutch prepositions) :)
Does that help? :)