The words opposite and across from mean that something is located at the other side of something.

The bank is opposite the post office. or The bank is across from the post office.

Henry sat opposite me at the table. or Henry sat across from me at the table.

The expression in front of simply means that one thing is closer to a point of reference (the "front" of an area, or the point of view of the person looking).

I stood in front of the bank.

He sat in front of me in the classroom.

The word before can be used for location/position, but it is somewhat rare and more formal. Instead, before is typically used for time or for position in a sequence:

We need to pack our bags before leaving for the airport.

The letter i comes before the letter e in the word "friend." or position in a sequence

Biggest things in the World

Fighter Plane

In 1992 a Major from the Israeli Air Force stole a WWII Mustang Fighter Plane. After spotting the plane in a reserve airfield where it was waiting to be moved to a museum, the major spent some time restoring it before he flew it the almost priceless vintage plane to Sweden where he sold it for $331,000! The plane is still being used by the Swedish companies and was only located when Interpol became involved!