The species: 
can act both as Bronsted acids and bases. For each case give the corresponding conjugate acid and base.
Bronsted acid: A bronsted acid is a substance that can donate a proton. A bronsted acid is proton-donor.
Bronsted base: A bronsted base is a substance that can accept a proton from an acid. A bronsted base is proton-acceptor.
When a base gain a proton, the residual part of it has a tendency to accept the proton. Therefore, it behaves as conjugate acid.
When an acid loses a proton, the residual part of it has a tendency to regain a proton. Therefore, it behaves as a conjugate base.
Conjugate base ⇌ Conjugate acid - H+
Conjugate base ⇌ Conjugate acid + H+
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