is by the length of; in a line with the length of; lengthwise next to while through
is from one side of an opening to the other.
Difference between “across” and “through”
walk across the tunnel
walk through the tunnel
Walk across the tunnel" simply implies that you started walking from one point and then crossed the tunnel. It doesn't say explicitly that you were inside the tunnel while walking. On the other hand "walk through the tunnel" explicitly tells that you were inside the tunnel while walking.
Difference between “along” and “across”
Along – is used to state someone or something is moving in one direction
We like to walk along the beach.
The city put up holiday decorations all along the street.
There are no lights along the highway, so driving at night is dangerous.
We like to drive along the sea.
Across – is used to state on the other side of someone or something
My sister lives across the street.
I have to go across the bridge to get to work.
My brother's room is across the hall from my room.
Do you go to school across from the park?