One brass plate is inserted between two charges. The force between two charges will

the net force between two charges will decrease, but the force on any charge due to other charge remains same. The net force decreases because an opposite field is produced due to polarisation of brass.

Inserting a brass plate will increase the force between two charges as brass is a conductor so it will be an equipotential surface and will not cause any effect on two charges. Implies that being a conductor it will decrease the distance between two charges and as F proportional to square of distance that`s why F will increase