In the following reaction;
HCO3- (aq) + H₂O(/)---> CO32- (aq) + H3O+ (aq)
HCO3 is a base and CO32- is its conjugate acid.
H₂O is a base and CO32- is its conjugate acid.
HCO3 is a base and H3O+ is its conjugate acid.
H₂O is a base and H3O* is its conjugate acid.