What are Prepositions
Prepositions are those words, which show the relationship of Nouns (or Pronouns) with the other words in the sentence. Prepositions are used before Nouns (or Pronouns) in the sentence.
Against is used to show what is opposite to the nouns or pronouns. Against is also used to show the relation or contact.
Against used as opposite:
Smoking at public places is against the law.
This policy is against the welfare of poor people.
India will play against Pakistan today.
She is against her sister.
People protested against that stupid law.
Against used to show the Contact or Relation
I made the payment against the bills.
What is the rate of exchange against Pound Sterling?
Move that table against the wall.
These bills are against your usages.
He parked the car against the main gate.