If you mean a sentence containing the word 'across', then it has several meanings which determine how it can be used.
Preposition : to or on the other side. He walked across the road.
Preposition : from one side to the other. The bridge stretched across the river.
Preposition : at an angle, He had deep cuts across his legs.

In the above three cases across can also be regarded as an adverb.

It can also relate to a crossword clue or answer. I finished nine across.

Hope this helps