Step-by-step explanation:
Unlike the traditional definition of defining acid as any substance that can react with a base to form salt and water, there are two other definition of acid.
These are lewis acid and bronsted lowry acid.
Lewis acid is any substance that can accept a pair of non bonding electrons. This is why H+ is able to as a lewis acid.
Bronsted lowry definition on the other hand is as follows; A brosted lowry acid is any species that can donate a proton to another molecule. A bronsted lowry base is any specie that can accept a proton from another molecule. NH3 is able to accept a proton, hence it is termed as a bronsted lowry base.
BF3 act as a lewis acid because it can accept accept a pair of non bonding electrons. Generally; an atom, ion, or molecule with an incomplete octet of electrons can act as an Lewis acid.