Across can be used as a preposition or an adverb. When used as a preposition, it is followed by a noun.
- We walked across the desert. (Here the noun the desert acts as the object of the preposition across.)
Across means from one side to the other.
- She walked across the road. (= She walked from one side of the road to the other side.)
Across can be used to talk about movement from one place, space or line to the other.
- The airplane flying across the Atlantic when it developed engine trouble.
- They are building a bridge across the river. (= The bridge will extend from one side of the river to the other side.)
Across can also be used to show the width of something.
- The river is only a few meters across in some places.
Across can show position. It means on the opposite side of a road, river, line etc.
- There is a petrol pump just across the road. (= The petrol pump is on the opposite side of the road.)
- The people across the border are moving. (= The people on the opposite side of the border are moving.)
Across can also be used to show position in many parts of an area or country.
- They have several offices across Europe. (= They have several offices in many parts of Europe.)
Across can also be used for saying that something affects a large part of your body.
- Suddenly he felt a sharp pain across his chest.
Across is also used for saying that something spreads until it covers the whole of a particular area.
- A warm smile spread across her face.
When used as an adverb, across is not followed by a noun.
- She hurried across to greet us. (Here the adverb across modifies the verb hurried.)