• Sam was sitting in front of my girlfriends in the cinema but behind my sister.
  
  
• I was waiting patiently in the queue. In front of me there were about two hundred people and behind me a further three hundred.

Before is normally used as a preposition to indicate time. Its opposite of which is after:

• Your brother arrived at the church shortly after three, but I distinctly remember saying to everyone: "You must be in your seats at or before three o' clock".
  
  
• Excuse me, I was here before you. I should therefore be in front of you in the queue.

However, before is used to refer to place when it indicates position in a list or when it means in the presence of somebody important:

• K comes before L in the alphabet, but after J.
  He had behaved so badly in school that he was brought before the headmistress.
  
  
• I was accused of dangerous driving but rather than pay the fine, I elected to appear before the local magistrates.

Note that in these last two examples before means facing and not one behind the other.

Before (conjunction or adverb)

Before is often used as a conjunction linking two clauses or as an adverb of time, meaning at some time before now.

• Give me a ring to let me know you are on your way before you leave the house.
  
  
• Make sure you get to the church before the bride arrives.
  
  
• Before she married Maurice, she went out with Austin for a couple of years.
  
  
• He was certain we had met before, but I was equally sure we hadn't, for I had never been there before.
  
  
• Within two minutes of it starting, I realized that I'd seen this film before.

Across (preposition)

In American English, across from as in across the road from me or across the table from me is expressed in British English by the prepositions opposite or facing:

• She sat facing me across the table. (She sat across the table from me.)
  
  
• They live directly opposite us in the green house. (They live across the road from us in the green house.)

In British English, across means from one side to the other, expressing movement, or on the other side of an imaginary line, expressing position:

• My older sister lives just across the road, but Jenny, my baby sister, lives right across the city, 60 minutes by Tube or two hours in the car.
  
  
• Rather than walk twenty miles to the nearest bridge, we decided to swim across the fast-flowing river, unaware of the dangerous currents.

Across or through?

Note the difference in use between across and through. Across suggests flat or open space, whereas through suggests a space which is closed with things on all sides:

• Although it was dark, I was not afraid of walking home through the forest.
  
  
• The ice was quite thick and he experienced no difficulty in skating right across the lake.
  
  
• We cycled across Bodmin moor and through a number of small villages.