When to use "between" and when to add an
in and say "in between" has become clear from the two very related SE posts: "In between" or "between"? and 'in between' or 'between'. Apparently the
in can always be omitted but is optional to add when "between" is used as a preposition for physical (and apparently also time-wise) comparisons:
There is a puddle [in] between us. Optional "in"
There is some tension between us. Don't use "in"
Question 1: Is this always correct? Are there never situations where the
in is mandatory?
Question 1a: And equivalently, in order for me to understand the proper usage: what difference does the word
in make in the sentence? Does it not make a difference? Can I safely omit the
in completely in any context and never bother with this again?
When I do choose to use the
in in relation to
between there apparently are several options:
The two first-mentioned seem well-explained in Should I use "in between" or "inbetween" in the following sentence?: the "in-between" is an adverb. But an answer in that post indicates that the latter form "inbetween" with no dash or space does exist.
Question 2: What exactly is the difference and meaning of these three versions? And do they all exist?