Thus we conclude that bicarbonate ion is a poorer base than hydroxide ion, and that carbonic acid a stronger acid that water. Alternately, having seen water and hydronium ion so many times we are probably aware that their pK a values are 15.7 and -17, respectively. Bicarbonate ion already has a negative charge. Removing a proton would give it a 2- charge. Upon deprotonation, carbonic acid would become bicarbonate ion with a single negative charge.
Any equilibrium favors the thermodynamically more stable side.
By replacing or adding no more than two atoms, change the structure of the strongest acid to make it even stronger. Draw the structure of the new acid here. There are many possible answers to this question. Two are presented here.