Pyridine molecules have been used to passivate surface Pb2+ sites of CH3NH3PbI3, to recrystallize CH3NH3PbI3, and to bleach CH3NH3PbI3. However, these results contradict each other, as recrystallization and optical-bleach require transformation of bulk CH3NH3PbI3, but surface passivation demands the confinement of the reaction at the surface region. The underlying mechanism for these seemly contradicting results is not yet understood. In this paper, we show, at 25 degrees C, partial pressure of pyridine vapor is a determining factor for its reaction behaviors with CH3NH3PbI3: one can modify the surface region of CH3NH3PbI3 by using pyridine vapor of pressure 1.15 torr or lower but can transform the whole bulk CH3NH3PbI3 film with a pyridine vapor of 1.3 torr or higher. Our result is the first demonstration that the reaction modes, i.e., surface-confined reaction and bulk transformation, are very sensitive to the partial pressure of under-saturated pyridine vapor. Despite the different reaction behaviors, it is interesting that in all pressure ranges, pyridinium ion is a main product from the reaction between pyridine and CH3NH3PbI3 the reaction between pyridine and CH3NH3PbI3. It is important to note 1.3 torr is much smaller than the saturated vapor pressure of pyridine (20 torr at 25 degrees C). These findings provide a guidance in applying pyridine and other amines to functionalize and transform CH3NH3PbI3 and other hybrid halide perovskites. It also highlights the critical role of fundamental studies in controllably modifying CH3NH3PbI3.