We do not normally use before to talk about position/place. Instead, we use in front of.
A tall guy was standing in front of me.
There were hundreds of people in front of me in the queue.
The professor stood in front of the desk.
The opposite of in front of is behind.
When the professor stands in front of the desk, the desk is behind him.
Who is that fat guy standing behind Jane?
As a preposition, before normally indicates time. It is the opposite of after.
I need to be there before 8 pm. (NOT I need to be there in front of 8 pm.)
The teacher told us that we should be in our seats at or before 9 am.
Before can refer to place in a few cases. For example, you can bring somebody before the magistrate. Students who misbehave in the class are often brought before the head master / mistress.
Before can also indicate position in a list.
The letter C comes before D and after B.
Before (conjunction or adverb)
Before can also be used as a conjunction or an adverb clause of time.
I should get to work before my boss arrives.
I will give you a ring before I leave.
Before she married Justin, she dated another guy for a couple of years.
Across is a preposition. In British English, across means ‘from one side to the other’.
He walked across the road.
Across can also mean on the opposite side of a road, line etc.
My sister lives across the road.