The words opposite and across from mean that something is located at the other
side of something.
▶️ The bank is opposite the post office.
= The bank is across from the post office.
= the bank is on the other side of the street from the post office
▶️ Henry sat opposite me at the table.
= 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.
= packing bags must happen first, leaving for airport must happen second
▶️ The letter i comes before the letter e in the word “friend.”
= position in a sequence