By introducing the metal plate between the plates of charged capacitor, the capacitance of capacitor increases
Reason: It is thickness of metal plate, then
$$ C= \dfrac{\varepsilon oA}{d-t(1-\dfrac{1}{k})} $$
For metal plate K= $$\infty $$, $$ C=\dfrac{\varepsilon oA}{(d-t)} $$