• When the teacher writes on the whiteboard, the students are behind him (or her).
• Who is that person behind the mask?
• I slowly down because there was a police car behind me.
Between
Between normally refers to something in the middle of two objects or things (or places).
• There are mountains between Chile and Argentina.
• The number 5 is between the number 4 and 6.
• There is a sea (The English Channel) between England and France.
Across From / Opposite