The strength of an acid is determined by its ability to donate a proton (H+). Strong acids completely dissociate in water, releasing all of their protons. Weak acids only partially dissociate in water, releasing only a small fraction of their protons.

HCOOH is a weak acid because it only partially dissociates in water. The equilibrium constant for the dissociation of HCOOH is very small, which means that very few HCOOH molecules actually dissociate. This means that HCOOH does not release a lot of protons into solution, and therefore it is not a strong acid.