The capacitance of A conductor is affected by the presence of A second conductor that is uncharged and isolated electrically. Why?

What happens is the potential value of the conductor decreases due to the presence of second conductor

as the capacitance is given by C = q/v

the value of v deceases as v-v1

thus the new capacitance is = C' = q/v-v1 thus the lowering of v increases the capacitance