Across and over can both be used to mean ‘on the other side of a line / road / bridge etc’.
- There is a hospital across / over the border. (= There is a hospital on the other side of the border.)
- His shop is across / over the road. (= His shop is on the other side of the road.)
Across and over can also be used to talk about movement to the other side of a line / road etc.
- He jumped across / over the stream.
We prefer over when we say ‘on/to the other side of something high’.
- He climbed over the wall. (NOT He climbed across the wall.)
We prefer across when we say ‘on / to the other side of something flat’.
- We swam across the river. (NOT We swam over the river.)
The preposition along is used with nouns that refer to things with a long thin shape. Examples are: road, river, corridor, line
- She ran along the road.
- There are trees along the riverside.
Along and through
To talk about periods or activities, we prefer through.
- She was silent all through the journey. (NOT She was silent all along the journey.)
Note that along can also be used as an adverb particle.
- Come along. (= Come with me.)
- Run along now. (= Go away.)